[media] bttv: convert to the control framework

Note that the private chroma agc control has been replaced with the standard
CHROMA_AGC control.
Also fixes a mute/automute problem where closing the file handle would force
mute on. That's not what you want since that would make the mute state out of
sync with the mute control. Instead check against the user count.

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