ASoC: wcd9320: Enable MBHC polling

There is a leakage in the wcd9320 codec's MBHC circuit that raises false
MBHC IRQs.  Prevent this leakage with override switch to avoid false IRQs
so codec driver can start MBHC polling.
MBHC polling is required to support headset button press and release
detection.

Change-Id: Ia2d7321ca834db770e1a8dc1ab5b0c80e5651ab7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>