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>