wlcore: call ieee80211_sched_scan_stopped on interface removal

The interface might go down before we got the SCHED_STOPPED
event, so make sure to call ieee80211_sched_scan_stopped()
if the scanned interface is removed.

Replace sched_scanning with sched_vif in order to save
the scanned interface.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>