ASoC: Allow WM8903 mic detect disable and don't force bias on

Don't force enable the microphone bias on WM8903 when doing jack
detection, and don't force enable microphone bias. This allows
platforms to only enable microphone detection when a jack has been
inserted.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>