[ALSA] ca0106: power down SPI DAC channels when not in use

For cards with an SPI DAC (SB Live 24-bit / Audigy SE), power down channels
0-2 when not in use.  They are powered up on PCM open and down again on PCM
close.  Channel 4 (== Front) is not powered down, as it is used for capture
feedback.  Powering it down would effectively kill line in pass-through.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>