ASoC: wcd9xxx-mbhc: Disable micbias for impedance detection

If micbias is enabled during impedance detection, it can result into pop
noise on headphone left and right channels. Since micbias is not really
used for impedance detection measurements, it is safe to disable the
micbias during impedance measurements.

CRs-fixed: 675563
Change-Id: Ibd8cf271cf56cffa405ca1fc3f42d7f0d3da4cb1
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>