wifi: mac80211: use wiphy locked debugfs helpers for agg_status

The read is currently with RCU and the write can deadlock,
convert both for the sake of illustration.

Make mac80211 depend on cfg80211 debugfs to get the helpers,
but mac80211 debugfs without it does nothing anyway. This
also required some adjustments in ath9k.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>