[media] usbvision: replace current_norm by g_std

current_norm use is deprecated because it is per-devicenode and if you
have more device nodes all dependent on the same video source, then this
no longer works. Just implement g_std instead.

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