V4L/DVB (12273): em28xx-video: rename ac97 audio controls to better document it

As em28xx chip has nothing to do with volume/mute controls, rename those
controls to properly indicate that they control the companion AC97 chip
that it is inside the boards with this chip.

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