ASoC: rsnd: tidyup DVC control method

DVC can use Volume and Mute control,
and these control methods doesn't have much difference.
This patch cleanup current method,
and it will be used for Mute control.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>