usb: dwc2: gadget: don't reset gadget's driver->bus

UDC driver should not touch gadget's driver internals, especially it
should not reset driver->bus. This wasn't harmful so far, but since
commit fc274c1e99 ("USB: gadget: Add a new bus for gadgets") gadget
subsystem got it's own bus and messing with ->bus triggers the
following NULL pointer dereference:

dwc2 12480000.hsotg: bound driver g_ether
8<--- cut here ---
Unable to handle kernel NULL pointer dereference at virtual address 00000000
[00000000] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in: ...
CPU: 0 PID: 620 Comm: modprobe Not tainted 5.18.0-rc5-next-20220504 #11862
Hardware name: Samsung Exynos (Flattened Device Tree)
PC is at module_add_driver+0x44/0xe8
LR is at sysfs_do_create_link_sd+0x84/0xe0
...
Process modprobe (pid: 620, stack limit = 0x(ptrval))
...
 module_add_driver from bus_add_driver+0xf4/0x1e4
 bus_add_driver from driver_register+0x78/0x10c
 driver_register from usb_gadget_register_driver_owner+0x40/0xb4
 usb_gadget_register_driver_owner from do_one_initcall+0x44/0x1e0
 do_one_initcall from do_init_module+0x44/0x1c8
 do_init_module from load_module+0x19b8/0x1b9c
 load_module from sys_finit_module+0xdc/0xfc
 sys_finit_module from ret_fast_syscall+0x0/0x54
Exception stack(0xf1771fa8 to 0xf1771ff0)
...
dwc2 12480000.hsotg: new device is high-speed
---[ end trace 0000000000000000 ]---

Fix this by removing driver->bus entry reset.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20220505104618.22729-1-m.szyprowski@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc2/gadget.c

@@ -4544,7 +4544,6 @@ static int dwc2_hsotg_udc_start(struct usb_gadget *gadget,
 
 	WARN_ON(hsotg->driver);
 
-	driver->driver.bus = NULL;
 	hsotg->driver = driver;
 	hsotg->gadget.dev.of_node = hsotg->dev->of_node;
 	hsotg->gadget.speed = USB_SPEED_UNKNOWN;