ASoC: mbhc: Add stub definition for mbhc deinit

Audio mbhc feature is not required for all the targets.
Add the stub version of wcd_mbhc_deinit to support target that
doesn't have CONFIG_SND_SOC_WCD_MBHC defined.

Change-Id: I0c312ac59ce689c8370f6c78e254e5713044572d
Signed-off-by: Deven Patel <cdevenp@codeaurora.org>