mac80211: Complete ampdu work schedule during session tear down

Commit 7a7c0a64 ("mac80211: fix TX aggregation start/stop callback race")
added a cancellation of the ampdu work after the loop that stopped the
Tx and Rx BA sessions. However, in some cases, e.g., during HW reconfig,
the low level driver might call mac80211 APIs to complete the stopping
of the BA sessions, which would queue the ampdu work to handle the actual
completion. This work needs to be performed as otherwise mac80211 data
structures would not be properly synced.

Fix this by checking if BA session STOP_CB bit is set after the BA session
cancellation and properly clean the session.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
[Johannes: the work isn't flushed because that could do other things we
 don't want, and the locking situation isn't clear]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>