[media] cx231xx: improve std handling

Set the initial standard of subdevices instead of leaving it undefined.
Also update the width and height when a new standard is chosen and
return -EBUSY when attempting to change the standard while videobuf is
busy.

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