ASoC: rt5677: use more of the volume range from DACs

The DACs volume can go over 0, both according to the data sheet and
real world testing.  The control can go up to +30dB.

This was tested by playing audio at full volume on a samus chromebook.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
Signed-off-by: Ross Zwisler <zwisler@google.com>
Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>