prima: Protect pHashTable with lock

lim_is_assoc_req_for_drop() uses pHashTable which can be accessed
by peDeleteSession simulataneously. This can lead to crash as
memory for pHashTable can be deleted.

Fix this by protecting usage of pHashTable with a lock.

Change-Id: Iaef7a26d9f3e1ccb76807c9dcf140a6f3de34d8e
CRs-Fixed: 2771345