ALSA: hda - Initialize vmaster slave volumes

When the driver is changed to use vmaster or a new slave element is
added by the improvement of the parser code, user may face often the
silent output because of the muted slave mixer although Master volume
is properly set.  And they complain.  And I get upset.

Although such a mixer element should be initialized via "alsactl init",
it'd be more user-friendly if the known output slaves are unmuted and
set to 0dB so that user can control the volume only with Master as
default.  Since Master is still set muted as default even with this
change, no risk of the speaker blow up, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>