urtwn: first cut of 11n

[Adrian Chadd]

HIya,

https://people.freebsd.org/~adrian/net80211/20160320-urtwn-11n-4.diff

* changed some "am I 11n node?" logic
* updated the default rate to be a fixed 11n rate if no rate control is present
* obey protmode for 11g and htprotmode for 11n rates.

And:

* verified that indeed yes, it is transmitting MCS packets, with
RTS/CTS from HT protmode, obeying the amrr rate control.

I've tested this on the 8188EU that uses amrr rate control; I haven't
validated the firmware rate control is working on the 8192CU.


-adrian