ASoC: wm8996: Put the microphone biases into bypass mode when idle

When we're not actively doing audio we don't need the microphone biases
to be regulated, noise is not important when we are not looking at the
audio signal. Save some power by putting the MICBIAS regulators into
bypass mode when not doing audio.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>