[media] em28xx: Remove unused AC97 register definitions

There is a specific header sound/ac97_codec.h that defines these.
All drivers should use it instead of rolling its own set of macros.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>