ALSA: ca0106: Create a nice spot for mapping channels to dacs.

This is to allow a future patch to have card specific mappings between
dacs, which is required since the Sound Blaster 5.1vx seems to have a
different mapping to what was previously used.

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