[media] cx231xx: fix vbi compliance issues

Various v4l2-compliance fixes: remove unused sliced VBI functions, zero the
reserved fields of struct v4l2_vbi_format and implement the missing s_fmt_vbi_cap.

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