[ALSA] soc - WM9712 PCM volume

This patch suggested by Joe Sauer adds PCM playback volume kcontrol for
the WM9712.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>