This was off because the net80211 aggregation code was using the same
state pointers for both fast frames and ampdu tx support which led to some
pretty unfortunate panic-y behaviour.
Now that net80211 doesn't panic, let's flip this back on.
It doesn't (yet) do the horrific sounding thing of A-MPDU aggregates
of fast frames; that'll come next. It's a pre-requisite to supporting
AMSDU + AMPDU anyway, which actually speeds things up quite considerably
(think packing lots of little ACK frames into a single AMSDU.)
Tested:
* QCA955x SoC, AP mode
* AR5416, STA mode
* AR9170, STA mode (with local fast frame patches)
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