ALSA: ca0106: Pull out dac powering routine into separate function.

This is ground work for a future commit where cards (such as the Sound
Blaster 5.1vx) have different mappings between dacs and channels.

Signed-off-by: Andy Owen <andy-alsa@ultra-premium.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>