ASoC: mc13783: Use SOC_ENUM_SINGLE_VIRT_DECL()

For the upcoming consolidation for MUXs and virtual MUXs we need to mark virtual
enums as such.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Philippe Rétornaz <philippe.retornaz@epfl.ch>
Signed-off-by: Mark Brown <broonie@linaro.org>