[ALSA] cmipci: reorganize set_dac_channels()

By reorganizing the code that sets the CHB3DxC bits we can not only
simplify this code but also fix the bug where the CHB3D8C bit was not
reset when playing a stereo stream after a 7.1 stream.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>