mac80211: fix race in TX aggregation

When disabling TX aggregation because it was rejected or from
the timer (it was not accepted), there is a window where we
first set the state to operation, unlock, and then undo the
whole thing. Avoid that by splitting up the stop function.
Also get rid of the pointless sta_info indirection in the timer.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>