ASoC: wcd9xxx: Avoid enabling button polling from event notify handler

During insertion, it is possible that recording is already
enabled from handset which will switch to headset. During
POST_MICBIAS_2_ON event, though button polling is not actually
enabled, due to polling_active becomes true, the event
notification handler pauses and re-starts button polling.
This could lead to unknown state for button polling state
machine.

CRs-Fixed: 639116
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>

Change-Id: I23eff6f9e3534fd36ee605312c160968f67d68a9