mac80211: don't clear sched_scan_sdata on sched scan stop request

ieee80211_request_sched_scan_stop() cleared
local->sched_scan_sdata. However, sched_scan_sdata
should be cleared only after the driver calls
ieee80211_sched_scan_stopped() (like with normal hw scan).

Clearing sched_scan_sdata too early caused
ieee80211_sched_scan_stopped_work to exit prematurely
without properly cleaning all the sched scan resources
and without calling cfg80211_sched_scan_stopped (so
userspace wasn't notified about sched scan completion).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>