ASoC: wcd9xxx: improve mbhc button release detection performance

MBHC hardware block monitors microphone's voltage when button is pressed
to detect button release event.  Reprogram release threshold to narrow
down the threshold voltage to be able to detect button release quickly.

CRs-fixed: 509925
Change-Id: I9802564ea5467f7b5b23a87191ded6ed2f10ab98
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>