[media] bttv: use centralized std and implement g_std

The 'current_norm' field cannot be used if multiple device nodes (video and
vbi in this case) set the same std.

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