qcacmn: Process FW stats event on VDEV_DOWN

FW sends vdev stats on vdev down, current host
implementation does not process this event as host is
only processing the stats events for which stats are
requested.
On vdev down FW sends the last available vdev stats and
based on that host updates it's RSSI on disconnect.
With CP stats infrastructure host is not processing
this event and is not updating rssi on disconnect.

To address above issue add a logic to process all the
vdev stats event received by host.

Change-Id: I9a5c32d061562cd3e3741cd9fe4a69f3278e7a98
CRs-Fixed: 2497104