qcacmn: Fix ref leak of vdev if scan is rejected

if gEnableDFSChnlScan is zero and scan channel list
contains only DFS channel, all DFS channels are
skipped and thus number of channel become zero.
ucfg_scan_start rejects scan request based on zero
number of channels and does not does not release
reference of vdev.

Fix is to add check and return failure before taking
the reference of the vdev.

Change-Id: I82ef308a335e6cbfd0d5a88e5c597f34b1cc8076
CRs-Fixed: 2427724