staging: xillybus: Use the return value of wait_event_interruptible

Rather than checking the wait condition, the return value of
wait_event_interruptible() and wait_event_interruptible_timeout() is
used.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>