wifi: cfg80211: fix locking in sched scan stop work

This should use wiphy_lock() now instead of acquiring the
RTNL, since cfg80211_stop_sched_scan_req() now needs that.

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