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>

include/asoc/wcd-mbhc-v2.h

@@ -1,5 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
-/* Copyright (c) 2014-2018, The Linux Foundation. All rights reserved.
+/* Copyright (c) 2014-2019, The Linux Foundation. All rights reserved.
  */
 #ifndef __WCD_MBHC_V2_H__
 #define __WCD_MBHC_V2_H__
@@ -540,6 +540,7 @@ struct wcd_mbhc {
 	bool gnd_swh; /*track GND switch NC / NO */
 	u32 hs_thr;
 	u32 hph_thr;
+	u32 micb_mv;
 	u32 swap_thr;
 	u32 moist_vref;
 	u32 moist_iref;