ASoC: wm8962: Export DAC/ADC monomix switches

This allows solutions like ALSA UCM to utilize hardware mono downmix
for cases where mono output to a single speaker is desired only in
specific situations (like on a mobile phone).

Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/3662154.EqNIRYjrc8@pliszka


Signed-off-by: Mark Brown <broonie@kernel.org>