NFC: Remove unneeded pn533 dev NULL check

container_of() works by subtracting the offset of the member.  The math
can't really return a zero here.  Sometimes people check it when they
actually meant to check something else but in this case we can just
remove the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>