ASoC: msm: qdsp6v2: Fix deadlock due to mutex unlock

Mutex is not unlocked in one of the failure case
in sound model buffer allocation function. This
causes deadlock if the failure condition occurs.
This change handles the mutex unlock correctly in
failure case.

Change-Id: I24b52c8db329d8b5fdd00b06a280b08a286907cc
Signed-off-by: Venkat Sudhir <vsudhir@codeaurora.org>