ASoC: wcd9xxx: improve mbhc button detection performance

Update mbhc ground voltage calibration selectively to be able to
accomplish accurate voltage measurement without compromising of measurement
latency.
Revise false button detection logic for different codecs to perform
false button checking for same amount of time.

CRs-fixed: 532971
Change-Id: Id5cb46336a83053ee9135461070ccb5ee58d6d90
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>