mac80211: fix A-MSDU aggregation with fast-xmit + txq

A-MSDU aggregation alters the QoS header after a frame has been
enqueued, so it needs to be ready before enqueue and not overwritten
again afterwards

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>