asoc: codecs: add a null pointer check in mbhc driver

Add a missing function pointer null check condition in
mbhc driver to avoid null access during fast
insertion-removal.

Change-Id: I6aa64325cea6f7aa551115969d1e6ba184ec8d14
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>