msm: adsprpc: Avoid double free on map

Decrement and check the ref count of map
inside the lock. Otherwise, two threads may
free the same map.

Change-Id: I081b937bfd3e8da3e2480f062cad6966662994b5
Acked-by: Sharad Kumar <sharku@qti.qualcomm.com>
Signed-off-by: Abhinav Parihar <quic_parihar@quicinc.com>
(cherry picked from commit 1f0408fc)