qcacmn: Handle gracefully if scheduler cb is not registered

Currently when firmware crashes during driver load,
message queue handlers for scheduler context are not
registered. While handling the FW down event, driver
posts the message to scheduler queue. Since the queue
handler is not registered, it results in crash.

To address this issue, handle the msg gracefully and
return error in case callback is not registered.

CRs-Fixed: 3251983
Change-Id: I5dd55e701cf332153dca1f6678271506b9690f6e