ASoC: Use generic control handlers for S8 control

Commit f227b88f ("ASoC: core: Add signed register volume control logic")
added support for signed control to the generic volsw control handler.
This makes it possible to use them for the S8 control as well, rather than
having to use a custom control handler implementation.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>