[urtwn] first cut of getting the fast-frames / amsdu support in shape.

The urtwn hardware transmits FF/A-MSDU just fine - it takes an 802.11
frame and will dutifully send the thing.

So:

• bump RX queue up from 1. Why's it 1? That's really silly.
• Add the "software A-MSDU" encap capability bit.
• bump the TX buffer size up so we can at least send A-MSDU frames.
• track active frames submitted to the NIC - we can't make assumptions about how many are in flight in the NIC though. For 88E parts we could use per-packet TX indication, but for R92 parts we can't. So, just fake it somewhat.
• Kick the transmit queue when we finish reception; try to avoid stalls.
• Kick the FF queue a little more regularly.

A-MSDU TX won't happen until the net80211 side is done, but atheros
fast-frames support should now work.

Tested:

• urtwn0: MAC/BB RTL8188EU, RF 6052 1T1R ; A-MSDU transmit.