[media] cx88: fix register mask

Ensure that the register is aligned to a dword, otherwise the read could
read out-of-range data.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>