During tx completion, tx_lock is held for longer than required, preventing
efficient refill of htt->pending_tx. Refactor the code so that only MSDU
related operations are protected by the lock.

Improves downstream performance on a dual-core ARM Freescale LS1024A
(f.k.a. Mindspeed Comcerto 2000) AP with a 3x3 client from 495 to 580 Mbps.
Other CPU bound multicore systems may also benefit.
Signed-off-by: NDenton Gentry <dgentry@google.com>
Signed-off-by: NAvery Pennarun <apenwarr@google.com>
[mfaltesek@google.com: removed conflicting code for tracking msdu_ids.]
Signed-off-by: NMarty Faltesek <mfaltesek@google.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>