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>