[media] cx231xx: remove current_norm usage

The use of this field is deprecated since it will not work when multiple
device nodes reference the same video input (the video and vbi nodes in
this case). The norm field should be a device-global value.

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