CRAS: mix - reorder exported functions

This will make it clearer when the scale and mix_add functions expand
to support formats other than s16le.

Change-Id: I28c6403e2c35d93164b8c6ea6dc758994bac1e81
Signed-off-by: Dylan Reid <dgreid@chromium.org>
CRAS: mix - Add 32 and 24 bit mixing

This will be used for devices that support S32_LE  and S24_LE formats.

BUG=chromium:219948
TEST=added unittests

Change-Id: I41f0f49ca8cc81e5321d3eed1767ded541355aea
Signed-off-by: Dylan Reid <dgreid@chromium.org>
CRAS: dev_stream - Mix proper format.

Mix the format that matches the device.  This adds a format parameter
to dev_stream_mix so the correct format can be used when mixing.

BUG=chromium:219948
TEST=play through USB DAC that supports 32 bit samples only.

Signed-off-by: Dylan Reid <dgreid@chromium.org>