staging: pi433: Return thread immediately when kthread_should_stop() call.

When kthread_stop() called by removing module, running thread should
return immediately. Otherwise, It is very dangerous that thread may access
any released data like struct pi433_device.
Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -736,7 +736,7 @@ pi433_tx_thread(void *data)
 					 device->free_in_fifo == FIFO_SIZE ||
 					 kthread_should_stop());
 		if (kthread_should_stop())
-			dev_dbg(device->dev, "ABORT\n");
+			return 0;
 
 		/* STOP_TRANSMISSION */
 		dev_dbg(device->dev, "thread: Packet sent. Set mode to stby.");