net80211: change ieee80211_ratectl_rate() to not return a rix

There are only a few places where the returned rix is used:

• linuxkpi - logging
• bwi/bwn - used for finding a fallback rate to choose, which honestly should be returned by the ratectl API
• iwm - building the rateset to program into firmware

Everyone else uses the dot11rate value in ni->ni_txnode.

This is a precursor for VHT and later rate support; where currently
there aren't rate tables in ieee80211_phy.c for VHT and later
rates.

Although it's likely doable to add tables for VHT, 11ax and MU-OFDMA
(HE) rates are sufficiently larger/different to just not fit in the
current scheme without more refactoring.

Differential Revision: https://reviews.freebsd.org/D48603
Reviewed by: bz, thj