LinuxKPI: 802.11: dump station_info using (*sta_statistics)() as sysctl

Implement the mac80211 (*sta_statistics) to obtain station statistics.
Fix up struct station_info for that.
Add supporting funtions to decode station_info and rate_info_bw fields
to make the output more usable to human beings.

Add a per-VIF (VAP) sysctl to export that information. This is mostly
used for now to see what the driver/fw thinks about its TX rate.
We should later gather this information in different ways and pass it
to net80211 to export it to user space so we can finally have nice
station information (also for the non-AP side).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days