qcacmn: Handle DFS AP's in scan result

if gEnableDFSChnlScan is zero and scan channel list
contains only DFS channel, all DFS channels are
skipped and thus number of channel become zero.
When scan req is sent to firmware with zero
channel, it scan all valid channels which contain
DFS channels as well.With gEnableDFSChnlScan set to
zero, no DFS channels should be scanned.

Fix is to reject the scan request if number of
channels become zero after filtering invalid
channels.

Change-Id: Ife5c0f0e3173197b43809f63ca95281d815a2c3c
CRs-Fixed: 2419126