ALSA: pcxhr - add support for pcxhr stereo sound cards (mixer part)

- add support for pcxhr stereo cards mixer controls
- adjust tlv db scales to real dBu values
- fix bug with monitoring volume control pcxhr_monitor_vol_put
- do some cleanup

Signed-off-by: Markus Bollinger <bollinger@digigram.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>