ALSA: Au88x0 - Support 4 channels playback when AC97 codecs has SDAC bit

- Check SDAC bit of AC97 codec for supporting 4 channels playback.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>