ASoC: cs42xx8: fix the noise in the right dac channel with mono playback

When playback mono wav with record in background, there will be some
nosie in the right dac channel. It seems that the ADC data has been
routed to the dac channel.

The cs42888 have 8 dac channels, it's appropriate to mute the unused
dac channels, and the noise will disappear.

Steps to reproduce this issue:
arecord -D hw:0,0 -f S16_LE -r 48000 -c 1 a.wav &
aplay -Dhw:0,0 audio48k16M.wav

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>