[ALSA] hda-codec - Fix conflict of Master volume in STAC92xx codec

The addition of volume knob as Master volume resulted in conflict with
the existing one by stac92xx_auto_create_hp_ctls().
This patch fixes the conflict, and still keeps the Master control for
codecs without volume knob as much as possible.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>