asoc: wcd937x: 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: I580e52e1bd225c2d99bfba4657d80670296709a0
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>