V4L/DVB (9648): em28xx: get audio config from em28xx register

Make use of the em28xx chip configuration register to determine whether
we have AC97 audio, I2S audio, or no audio support at all.

Thanks for Ray Lu from Empia for providing the em2860/em2880 datasheet.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>