ASoC: wcd9xxx: Fix mbhc voltage measurement inconsistency

There is mbhc voltage measurement inconsistency due to offset of
measurement result between with BG and without BG.
Change mbhc BG mode to slow mode and enable it while it's needed so
different BG mode won't make measurement offset.

Change-Id: I507aee6e00d81263a6368cd5727f2e63705d3ec6
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
4 files changed