[media] bttv: apply mute settings on open

Previously, this has been done implicitly for video device nodes by calling
set_input() (which calls audio_input() and also modified the mute
setting).
Since input and mute setting are now untangled (as much as possible), we need to
apply the mute setting with an explicit call to audio_mute().
Also apply the mute setting when the radio device node gets opened.

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>