[media] cx18: fix register range check

Ensure that the register is aligned to a dword, otherwise the range check
could fail since it assumes dword alignment.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>