rtwn: add placeholder for the per-MACID rate report

Some chipsets (such as the RTL8188E) have firmware which supports
a second kind of TX report - instead of a per-packet TX report,
it can generate a per-MACID summary of packet success/failure counters.

This would be helpful for those chips to cut back on the USB traffic
to get rate control feedback for the driver based rate control we're
currently using.

This is a no-op; it just gets the pieces in place for future work.

Differential Revision: https://reviews.freebsd.org/D47894