ASoC: twl6040: Use virtual DAPM mixer controls

By using the new virtual DAPM mixer controls it is possible to remove the
twl6040 specific implementation of virtual controls.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>