Staging: pi433: check error after kthread_run()

Error should be checked with IS_ERR after calling kthread_run()
instead of comparing the returned pointer to an int.

Found by sparse warning:

incompatible types for operation (<)
    left side has type struct task_struct *tx_task_struct
    right side has type int
Signed-off-by: Joseph Wright <rjosephwright@gmail.com>
Reviewed-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Tested-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -1152,7 +1152,7 @@ static int pi433_probe(struct spi_device *spi)
 	device->tx_task_struct = kthread_run(pi433_tx_thread,
 					     device,
 					     "pi433_tx_task");
-	if (device->tx_task_struct < 0)
+	if (IS_ERR(device->tx_task_struct))
 	{
 		dev_dbg(device->dev, "start of send thread failed");
 		goto send_thread_failed;