mac80211: update A-MPDU flag on tx dequeue

The sequence number counter is used to derive the starting sequence
number. Since that counter is updated on tx dequeue, the A-MPDU flag
needs to be up to date at the tme of dequeue as well.

This patch prevents sending more A-MPDU frames after the session has
been terminated and also ensures that aggregation starts right after the
session has been established

Fixes: bb42f2d1 ("mac80211: Move reorder-sensitive TX handlers to after TXQ dequeue")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>