wlan: Error handling while issuing roam offload scan req

In case if session id does not exist for a given BSSID, Driver
should not issue a roam offload scan request.

Fix is to make sure driver issue roam offload scan request only
if valid session-id exist for a BSSID.

Change-Id: Iab3d01b203ff3d70f45f2e77d69a761f7a956e86
CRs-Fixed: 2728005