ALSA: wm8990: Implement speaker volume PGA

The latest revisions of the WM8990 provide a programmable gain amplifier
for the speaker - configure the register cache and implement controls
for this. Older revisions of the device ignore writes to these controls.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>