There are a number of mixer controls that support negative values. They
use signed values for this with different number of bits for the values.
Currently they only support the positive range.

This patch replaces the unsigned mixers with signed mixers to support
the full range.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>