Add initial 11n support to if_rsu.

• Add a tunable to enable 11n if it's available, so to not anger people who upgrade.
  
  kenv hw.usb.rsu.enable_11n=1 before inserting the device.

• Add initial 11n htconfig bits;
• Enable 40MHz mode if it's available;
• Add 11n channels;
• Set 11n bits in the firmware.

It works for RX; I haven't tested TX aggregation just yet.
However the firmware doesn't do RX re-ordering, so I have to tie it into
the net80211 A-MPDU RX reorder path before I flip this on by default.

I've verified that I'm indeed actually seeing MCS 0->7 rates being received.
I haven't dug into whether it's actually transmitting 11n rates; I'll dig into
that later.