Make iwn, iwm, and iwx keep track of beacon parameters at run-time.

Make iwn, iwm, and iwx keep track of beacon parameters at run-time.

- HT protection settings (this was already implemented)
- ERP (11g) protection setting
- short slottime setting
- short preamble setting
- EDCA (QoS) parameters

All of these parameters are communicated in beacons and hardware is
now kept up-to-date with them.

Prompted by a problem report from Christian Ehrhardt regarding ERP.

Tested:
iwn 6205: stsp, Josh Grosse
iwm 7265: trondd
iwm 8265: stsp, Matthias Schmidt
iwm 9260: phessler
iwx ax200: stsp, jmc, gnezdo