V4L/DVB (12274): em28xx-video: better implement ac97 control ioctls

In the past, some devices with saa711x had their parameters controlled
directly inside em28xx driver, instead of using their proper module for
it.

Due to that, the ac97 controls were mixed with saa711x ones.

Older patches removed all saa711x controls, but we still need to control
ac97 devices on em28xx, since we don't have a separate v4l2 device for
it.

The proper way to address is to create a separate ac97 v4l2 device.
While we don't have it, we should clean up the code to allow having a
better view of what is part of em28xx core code and what's due to ac97
control inside it.

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