ASoC: wm9713: Use virtual control instead of virtual register

The wm9713 currently implements the virtual control for the Mic B Source MUX
using a virtual register. Replace this by using SOC_ENUM_SINGLE_VIRT().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>