asoc: wcd938x: initialize micb lock mutex

Micb lock which is used to synchronize micbias
control api is not initialized causing spin lock
errors. Initialize the mutex to avoid the issue.

Change-Id: I98c50fec8e8531333de643a85433d3e5973bfd14
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>