mac80211: Fix sending ADDBA response for an ongoing session

In case an ADDBA request is received while there is already
an ongoing BA sessions with the same parameters, i.e., update
flow, an ADBBA response with decline status was sent twice. Fix it.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>