ASoC: wcd: keep micbias enabled for headset

If micbias is disabled after recording is stopped,
buttons won't work. Dont disable micbias, instead
keep it on till headset is inserted and disable
when headset is removed.

CRs-Fixed: 653668
Change-Id: Id601355921abf91d24d3890bf280a961bb4f3479
Signed-off-by: Simmi Pateriya <simmip@codeaurora.org>