ALSA: ac97: Implement channel map workaround for ALC650

ALC650 has a channel swap option between surround and CLFE channels,
so we need to tweak the channel maps dynamically depending on the
register bit.

Now struct snd_ac97 can contain chmap pointers for playback and
capture.  The driver may store these and let ac97 driver changing the
channel mapping dynamically.

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