ALSA: hda: STAC_VREF_EVENT value change

Changed value for STAC_VREF_EVENT from 0x40 to 0x00 because the
unsol response value is only 6-bits width and the former value
was 1<<6 which is an overrun.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>