[ALSA] soc - Add 'Mono Playback Switch' to WM9712 codec driver

The following patch adds 'Mono Playback Switch' control to WM9712 codec
SoC driver.
Also, it fixes Treble, Bass and Mono playback volume inversion bits.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>