ASoC: wcd-mbhc: correct special headset detection logic

When detecting special headset, vref for special headset is
set based on headset threshold reference 1800mv or based on
regular micbias settings from device tree. Scale threshold
reference as per the special headset micbias voltage.

Change-Id: Ic4cc24e18efeb420b53a2154707c9399eb886181
Signed-off-by: Meng Wang <mwang@codeaurora.org>