wifi: mac80211: move sched-scan stop work to wiphy work

This also has the wiphy locked here then. We need to use
the _locked version of cfg80211_sched_scan_stopped() now,
which also fixes an old deadlock there.

Fixes: a05829a7 ("cfg80211: avoid holding the RTNL when calling the driver")
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>