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
Change-Id: I51ebf1d546aa59e219ce71517970b35730589b6c
Signed-off-by: Simmi Pateriya <simmip@codeaurora.org>