[media] au0828: add try_fmt_vbi support, zero vbi.reserved, pix.priv

Also get rid of unnecessary format type check.

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