cfg80211: add cfg80211_sched_scan_stopped_rtnl

Add locked-version for cfg80211_sched_scan_stopped.
This is used for some users that might want to
call it when rtnl is already locked.

Fixes: d43c6b6e ("mac80211: reschedule sched scan after HW restart")
Cc: stable@vger.kernel.org (3.14+)
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>