ALSA: usb-audio: Define registers for CM6206

The register map for CM6206 is known and we can define what
the values written to the different registers actually mean.

I tested to print the hex values before/after this change,
there is no functional change.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>