platform/msm: fix audio codec function pointer update

Fix the audio codec function pointer update by validating the
existing pointer before attempting to clear it. This check is
necessary in order to avoid a possible null deference during
the cable disconnect use case when the audio codec has not
been previously registered.

Change-Id: I7b3bd3dd4a83f335799f960a68391c272af256e0
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>