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>