ASoC: cros_ec_codec: extract DMIC EC command from I2S RX

Extract DMIC EC command from I2S RX.  Setting and getting
microphone gains is common features.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Acked-By: Benson Leung <bleung@chromium.org>
Link: https://lore.kernel.org/r/20191014180059.03.I93d9c65964f3c30f85a36d228e31150ff1917706@changeid


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