mac80211: fix stopping RX BA session from timer

Kalle reported that his system deadlocks since my
recent work in this area. The reason quickly became
apparent: we try to cancel_timer_sync() a timer
from within itself. Fix that by making the function
aware of the context it is called from.

Reported-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>