V4L/DVB (9651): em28xx: Improve audio handling

This patch properly implements audio handling on em28xx. Before this
patch, it was assumed that every device has an Empia 202 audio chip.
However, this is not true.

After this patch, specific AC97 chipset setup and configurations can be
done.

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