ASoC: wcd9xxx: Synchronize button press with notifiers

Button press handler and event notification handler needs
to be syncronized so that they do not interfere in the
excution leading to incorrect register writes or threshold
updates.

CRs-Fixed: 637805
Signed-off-by: Simmi Pateriya <simmip@codeaurora.org>

Change-Id: I982b6be65a44080f307417df9aa2868c23744ff6