[media] bttv: separate GPIO part from function audio_mux()

Move the GPIO part of function audio_mux() to a separate function
audio_mux_gpio().
This prepares the code for the next patch which will separate mute and input
setting.

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>