net80211: refactor amrr_update() into HT and legacy paths

Split the amrr_update() routine into HT and legacy paths.
Currently they're the same, minus HT / non-HT specific comments
and some logging changes.

HT rates don't monotonically go up (ie, going MCS 7 -> MCS 8 is wrong,
as MCS8 is "just" two stream MCS0) and failing a rate by a little
shouldn't necessarily preclude testing the next rate up.

This should be a no-op, besides the logging changes.

Locally tested:

• RTL8192CU, STA mode

Differential Revision: https://reviews.freebsd.org/D48248
Reviewed by: bz, thj