rtwn: bring the r92c rate control setup selection in line with tx descriptors

The rate control message was doing 11g+11n without 11b rates, but
the TX descriptor setup supports also falling back on 11b rates
when doing multi-rate retry / per-descriptor rate control.

So, line them up. They're not exactly the same as the TX path
supports pure-N and pure-G modes which the rate control configuration
does not, but there'll need to be a lot more work on supporting
those operating modes anyway (around things like self-generated
frame rate control/masks, beacon config, RTS/CTS selection, etc.)

Locally tested:

• RTL8192CU, STA mode

Differential Revision: https://reviews.freebsd.org/D48081
Reviewed by: bz