This changes the transmit rate control code to do a few things:
* use fixed rates (mcast, ucast, mgmt) where required.
* Don't use a hard-coded 11a or 11bg rate for non-data frames -
use what net80211 says we should use.
* use mgmtrate for EAPOL frames.
Reviewed by: avos
Differential Revision: https://reviews.freebsd.org/D7994