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: If9153f32a0ef126692185593e44df4778a7ef9ba
Acked-by: Abhishek Singh <abhishes@qti.qualcomm.com>
Signed-off-by: Santosh Sakore <quic_ssakore@quicinc.com>