Flip on half/quarter rate support.

Flip on half/quarter rate support.

No, this isn't HT/5 and HT/10 support.  This is the 11a half/quarter
rate support primarily used by the 4.9GHz and GSM band regulatory
domains.

This is definitely a work in progress.

TODO:

* everything in the last commit;
* lots more interoperability testing with the AR5212 half/quarter rate
  support for the relevant chips;
* Do some interop testing on half/quarter rate support between _all_
  the 11n chips - AR5416, AR9160, AR9280 (and AR9285/AR9287 when 2GHz
  half/quarter rate support is coded up.)