mac80211: release sched_scan_sdata when stopping sched scan

Assuming sched_scan_stop operation is synchronous the driver may not
necessary call ieee80211_sched_scan_stopped_work. Since this work is
the only place where sched_scan_sdata is released we can possibly run
into situation when it is never released. Fix this by releasing it
just after calling drv_sched_scan_stop.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>