qcacmn: If scan is for single channel don't filter the channel

Scan is rejected if single DFS channel is requested with
gEnableDFSChnlScan set to 0.

Fix is to allow single DFS channel scan and not to filter the
DFS channels.

Change-Id: Ifea85c7c85d3ace6cf08984c8c8e8ef00623303c
CRs-Fixed: 2454535