[ALSA] ac97 - Remove ac9_enum definition from public header

Modules: AC97 Codec

Remove the definition of ac97_enum struct from the public ac97_codec.h.
It's used only in the module.

The location of struct ac97_pcm is moved closer to its accessor
to improve readability.

Signed-off-by: Takashi Iwai <tiwai@suse.de>