qcacld-3.0: acquire lock before update connection list

Acquire connection list lock in policy mgr before update the
entry. The "conn_index" maybe changed for a connection entry
if other connection is up or down.
Fix by acquire connection to protect the whole "update" operation.

Change-Id: I91e82e74884ef32e83e0c4105e88bafe8d99db3d
CRs-Fixed: 2848209