[media] mcam-core: replace current_norm by g_std

The current_norm field is deprecated, replace this 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>