[media] via-camera: replace current_norm by g_std

The current_norm field is deprecated. Replace it by properly implementing
g_std.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>