[media] bttv: audio_mux(): use a local variable "gpio_mute" instead of modifying the function parameter "mute"

Function audio_mux() actually deals with two types of mute: gpio mute and
subdevice muting.
This patch claryfies the meaning of these values, but mainly prepares the code for
the next patch.
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>