ALSA: ac97: split out the generic ac97 registers

Split out from the ac97_codec.h the ac97 generic registers, which can be
used by a codec, typically a generic ac97 codec, and by the ac97 bus, to
scan an ac97 AC-Link.

This split encompasses all the AC97 standard registers, but not the
codec specific ones.

In order to have a clean split between former ac97 bus implementation
and the new coming one in sound/ac97, it is safer to not include any
former ac97 includes, excepting in sound/ac97/compat.c.

Amongst the thing to isolate :
 - don't have the struct snd_ac97 in sound/ac97/* (except compat.c) to not
   be "fooled" by a definition which would come with ac97_codec.h by
   "chance".
 - don't have to have snd_a97_*() functions, as they rely on struct
   snd_ac97.
  - don't want the struct snd_ac97_bus_ops, there is a new one

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>