ASoC: wcd: correct handling of MICBIAS regulator

8916 uses same LDO for both USB and MICBIAS,
so request for LDO on insertion of headset and
release it during removal of headset.

CRs-Fixed: 699095
Change-Id: Iac8cd61453007f1a1b9a9e4e9466f5dd698dec4e
Signed-off-by: Yeleswarapu Nagaradhesh <nagaradh@codeaurora.org>