serial: max310x: Use level-triggered interrupts

I was getting this trace along with a disabled IRQ when I was generating
heavy traffic over four daisy-chained UARTs (MAX14830) on my test kit
(Marvell Armada AM388, Solidrun Clearfog Base):

    irq 51: nobody cared (try booting with the "irqpoll" option)
    CPU: 0 PID: 68 Comm: irq/51-spi1.2 Not tainted 4.14.4 #7
    Hardware name: Marvell Armada 380/385 (Device Tree)
    [<c0110ba4>] (unwind_backtrace) from [<c010c1d8>] (show_stack+0x10/0x14)
    [<c010c1d8>] (show_stack) from [<c07776ac>] (dump_stack+0x84/0x98)
    [<c07776ac>] (dump_stack) from [<c016bdfc>] (__report_bad_irq+0x28/0xcc)
    [<c016bdfc>] (__report_bad_irq) from [<c016c204>] (note_interrupt+0x28c/0x2dc)
    [<c016c204>] (note_interrupt) from [<c01695d4>] (handle_irq_event_percpu+0x4c/0x58)
    [<c01695d4>] (handle_irq_event_percpu) from [<c0169624>] (handle_irq_event+0x44/0x68)
    [<c0169624>] (handle_irq_event) from [<c016ce80>] (handle_edge_irq+0x12c/0x1dc)
    [<c016ce80>] (handle_edge_irq) from [<c016872c>] (generic_handle_irq+0x24/0x34)
    [<c016872c>] (generic_handle_irq) from [<c03fc5a0>] (mvebu_gpio_irq_handler+0xe0/0x184)
    [<c03fc5a0>] (mvebu_gpio_irq_handler) from [<c016872c>] (generic_handle_irq+0x24/0x34)
    [<c016872c>] (generic_handle_irq) from [<c0168c4c>] (__handle_domain_irq+0x5c/0xb4)
    [<c0168c4c>] (__handle_domain_irq) from [<c0101520>] (gic_handle_irq+0x4c/0x90)
    [<c0101520>] (gic_handle_irq) from [<c010ce4c>] (__irq_svc+0x6c/0x90)
    Exception stack(0xeea77c30 to 0xeea77c78)
    7c20:                                     0000000a 018cba80 0000000a f098f680
    7c40: 0000020a f098f680 00000008 0000020a 018cba80 00000001 ee9302a0 eea76000
    7c60: ef2b2640 eea77c80 c050687c c0506894 80070013 ffffffff
    [<c010ce4c>] (__irq_svc) from [<c0506894>] (orion_spi_setup_transfer+0x118/0x20c)
    [<c0506894>] (orion_spi_setup_transfer) from [<c05069ac>] (orion_spi_transfer_one+0x1c/0x26c)
    [<c05069ac>] (orion_spi_transfer_one) from [<c05060e4>] (spi_transfer_one_message+0xec/0x500)
    [<c05060e4>] (spi_transfer_one_message) from [<c05059a4>] (__spi_pump_messages+0x3f4/0x680)
    [<c05059a4>] (__spi_pump_messages) from [<c0505e38>] (__spi_sync+0x1fc/0x200)
    [<c0505e38>] (__spi_sync) from [<c0505e60>] (spi_sync+0x24/0x3c)
    [<c0505e60>] (spi_sync) from [<c0505f48>] (spi_write_then_read+0xd0/0x17c)
    [<c0505f48>] (spi_write_then_read) from [<c0482efc>] (_regmap_raw_read+0xb0/0x250)
    [<c0482efc>] (_regmap_raw_read) from [<c04830c0>] (_regmap_bus_read+0x24/0x4c)
    [<c04830c0>] (_regmap_bus_read) from [<c04826f4>] (_regmap_read+0x60/0x148)
    [<c04826f4>] (_regmap_read) from [<c0482818>] (regmap_read+0x3c/0x5c)
    [<c0482818>] (regmap_read) from [<c04592b4>] (max310x_port_irq+0x104/0x2dc)
    [<c04592b4>] (max310x_port_irq) from [<c0459a40>] (max310x_ist+0x68/0xc0)
    [<c0459a40>] (max310x_ist) from [<c016a610>] (irq_thread_fn+0x1c/0x54)
    [<c016a610>] (irq_thread_fn) from [<c016a8d8>] (irq_thread+0x12c/0x1f0)
    [<c016a8d8>] (irq_thread) from [<c013e560>] (kthread+0x128/0x158)
    [<c013e560>] (kthread) from [<c0107a50>] (ret_from_fork+0x14/0x24)
    handlers:
    [<c0169694>] irq_default_primary_handler threaded [<c04599d8>] max310x_ist
    Disabling IRQ #51

On a multi-UART max310x, each UART has its own interrupt status register
which automatically de-asserts the IRQ line upon read. (There are also
top-level IRQ indicator registers which are not clear-on-read, but they
are not relevant here.) It was quite possible to receive a pending IRQ
for, e.g., UART0, enter the threaded IRQ handler, clear the ISR for
UART0 which de-asserts the IRQ line, and then race with another event on
the same chip, but a different UART channel. That resulted in another
edge on the shared-within-the-chip IRQ line which got intercepted by the
kernel.

That all led to an edge-level interrupt which was not being handled by
anybody because our threaded handler hasn't finished yet. As the chip
actually uses *level* triggered IRQs, let's convert the example DT
bindings to these.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/maxim,max310x.txt

@@ -30,7 +30,7 @@ Example:
 		clocks = <&clk20m>;
 		clock-names = "osc";
 		interrupt-parent = <&gpio3>;
-		interrupts = <7 IRQ_TYPE_EDGE_FALLING>;
+		interrupts = <7 IRQ_TYPE_LEVEL_LOW>;
 		gpio-controller;
 		#gpio-cells = <2>;
 	};