ALSA: hda - initialize node 0x21 properly on AD1988 codecs

The widget node 0x21 should be initialized as unmuted/full (0dB)
as default.  This will reduce additional manual work by user at the
first time use.

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