[media] vivid: do not allow video loopback for SEQ_TB/BT

Sequential top-bottom/bottom-top fields are not supported as video loopback.
This is too much work to implement for field settings that are rarely used.

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