ALSA: pcxhr - add support for pcxhr stereo sound cards (core change)

- Add support for pcxhr stereo cards
- minor bugfixes : period and buffer size consraints
- fix PLL register values
- do some clean up

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