staging: pi433: fix potential null dereference

Add a test for successful call to cdev_alloc() to avoid
potential null dereference. Issue reported by smatch.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Fixes: 874bcba6

 ("staging: pi433: New driver")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>