staging: pi433: add missing call to cdev_del()

If cdev_add() fails, cdev_del() should be called.
Add the missing cdev_del() call as pointed out by
Dan Carpenter.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>