[media] bttv: audio_mux(): do not change the value of the v4l2 mute control

There are cases where we want to call audio_mux() without changing the value of
the v4l2 mute control, for example
- mute mute on last close
- mute on device probing

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>