[media] bttv: fix field handling inside TRY_FMT

- don't return -EINVAL for invalid field types, handle those as if it
  was FIELD_ANY.
- the handling of FIELD_SEQ_BT/TB was wrong as well: if such field formats
  aren't supported, then fall back to FIELD_ANY instead of returning an error.

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