spi: Add cond_resched() in main message processing loop

When a controller has only PIO support it is very likely that we will
run into use cases where we spend a very large amount of time consuming
CPU.  Code that does this should call cond_resched() every once in a
while to give other tasks more of a chance to run so do that in the main
SPI loop, the overhead is negligable if it's not needed.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1184,6 +1184,9 @@ static void __spi_pump_messages(struct spi_master *master, bool in_kthread)
 		return;
 	}
 	mutex_unlock(&master->bus_lock_mutex);
+
+	/* Prod the scheduler in case transfer_one() was busy waiting */
+	cond_resched();
 }
 
 /**