[media] cx88: each device node gets the right controls

radio only sees audio controls, video sees video and audio and vbi
sees none.

Also disable the chroma_agc control if secam is selected.

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