ASoC: Manage WM8731 ACTIVE bit as a supply widget

Now we have supply widgets there's no need to open code the handling of
the ACTIVE bit.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Liam Girdwood <lrg@ti.com>