[media] bttv: do not save the audio input in audio_mux()

We can't and do not save the mute setting in function audio_mux(), so we
should also not save the input in this function for consistency.

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>