qcacmn: Enqueue scan request only after scan scheduling is success

Currently, vendor scan request is enqueued in the scan queue first and
then posted into the scheduler queue. Now if there is an SSR in between,
then scan request will be freed twice. Once in SSR context and again
in the caller context because scan scheduling fails due to user disabled
the scan.

To resolve this, post the scan request in scheduler queue first. Once
scan request scheduling is success, then enqueue the scan request in
scan queue. This approach can lead to a race condition like after
scheduling scan request, host may get scan response from firmware even
before the scan request is queued. To avoid this race, scan request
scheduling and enqueue should be protected by scan_req_q_lock.

Change-Id: Ic26da0181fb9a902a8a4abb8f68d9fc668f1b3f2
CRs-Fixed: 2714495