ASoC: tas2770: Allow mono streams

The part is a mono speaker amp, but it can do downmix and switch between
left and right channel, so the right channel range is 1 to 2.

Fixes: 1a476abc

 ("tas2770: add tas2770 smart PA kernel driver")
Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
Link: https://lore.kernel.org/r/20220808141246.5749-3-povik+lin@cutebit.org
Signed-off-by: Mark Brown <broonie@kernel.org>