ALSA: opti92x: use PnP data to select Master Control port

The Master Control port (MC) is available as the last
PnP resource (OPT005). Use this value instead fo guessing.

Also, add some comments to the code.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>