ALSA: Au88x0 - Implement subdevice volume controls

- add "PCM Playback Volume" controls for 16 playback subdevices
  This allow application to change the volume of each subdevice
  by using hardware mixer of au88x0 and default is zero gain/attenunation.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>