qcacld-3.0: Hold lock while iterating through scan queue

In wlan_serialization_is_cmd_in_vdev_list driver gets the node
and then operate on it without holding lock. It may happen that in
parallel other thread may delete the node. And thus in
wlan_serialization_is_cmd_in_vdev_list driver end up accessing freed
node.

Fix is to hold lock for full opration while going through the
scan queue.

Change-Id: I5bab8eba6ba6da9ad29137030e3651a400f71edd
CRs-Fixed: 2426873