rsu(4): Add support for 1T2R and 2T2R NICs.

This logic is mostly crimed from the reference driver and the linux
r92su driver.

I verified that it (a) worked on the rsu hardware I have, and (b)
did traffic testing whilst watching what ath(4) sent as a hostap.
It successfully sent MCS8..15 rates (which requires 2-stream reception)
as well as MCS0..7 (which is 1-stream.)

Tested:

• RTL8712, 1T1R NIC, MCS rates 0..7.
• RTL8712, 1T2R NIC, MCS rates 0..15

TODO:

• Find a 2T2R NIC!