[media] em28xx-alsa: add mixer support for AC97 volume controls

Export ac97 volume controls via mixer.

Pulseaudio will probably handle it very badly, as it has
no idea about how volumes are wired, and how are they
associated with each TV input. Those wirings are
card model dependent, and we don't have the wiring mappings
for each supported device.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>