In associated state, when bringing an interface down, existing
BA sessions are torn down. When this is in progress, nothing
prevents mac80211 from accepting another BA session start request.

Use a new station flag to fix this.
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>