ALSA: x86: Cache AUD_CONFIG register value

At enabling the audio, we modify AUD_CONFIG register bit 0.  So far,
it does read-modify-write procedure with a special hack for the
channel bits due to the silicon bug.  But we can optimize it by
remembering the AUD_CONFIG register value privately.  This simplifies
the things a lot.

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