ASoC: audio: fix unlock Null pointer issue

-In afe_open, trying to unlock mutex on variable
that is already freed.
-correct the sequence by unlock mutex first then
free the variable.

CRs-Fixed: 423196
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>
(cherry picked from commit 4c120ae85c6c5ac2adfdf61956b45d02461a93e3)

Change-Id: I6c29a58e7138d29fe1a5c0641be795f6f4fbbd47
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>