ASoC: cros_ec_codec: read max DMIC gain from EC codec

Read max DMIC gain from EC codec instead of DTS.  Also removes the
dt-binding of max-dmic-gain.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20191017213539.05.Id4657c864d544634f2b5c1c9b34fa8232ecba44d@changeid


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