ASoC: cs42l42: Fix mono playback

ASoC: cs42l42: Fix mono playback

I2S always has two LRCLK phases and both CH1 and CH2 of the RX
must be enabled (corresponding to the low and high phases of LRCLK.)
The selection of the valid data channels is done by setting the DAC
CHA_SEL and CHB_SEL. CHA_SEL is always the first (left) channel,
CHB_SEL depends on the number of active channels.

Previously for mono ASP CH2 was not enabled, the result was playing
mono data would not produce any audio output.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: 621d65f3b868 ("ASoC: cs42l42: Provide finer control on playback path")
Link: https://lore.kernel.org/r/20210805161111.10410-4-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>