[ath] modify cabq and per-node packet usage limits.

• limit cabq to 64 - in practice if this stays at ath_txbuf then all buffers can be tied up by a very busy broadcast domain (eg ARP storm, way too much MDNS/NETBIOS). It's been like this in the freebsd-wifi-build AP project for the longest time.

• Now that I figured out the hilarity inherent in aggregate forming and AR9380 EDMA work, change the per-node to 64 frames by default. I'll do some more work to shorten the queue latency introduced when doing data so TCP isn't so terrible, but it's now no longer /always/ tens of milliseconds of extra latency when doing active iperf tests.

Notes:

The reason for the extra latency is partly tx/rx taskqueue handling and
scheduling, and partly due to a lack of airtime/QoS awareness of per-node
traffic. Ideally we'd have different limits/priorities on the QoS/TID
levels per node so say, voice/video data got a better share of buffer
allocations over best effort/bulk data, but we currently don't implement
that. It's not /hard/ to do, I just need to do it.

Tested:

• AR9380 (STA), AR9580 (hostap) - both with the relevant changes. TCP is now at around 180mbit with rate control and RTS protection enabled. UDP stays at 355mbit at MCS23, no HT protection.