ath5k: Minor QCU updates

* Sync qcu.c with legacy-hal
* Add some more comments
* Set QCU mask to save power (QCU mask controls which QCUs are attached
  to each DCU, we do a 1:1 mapping)

TODO: Use max QCU from EEPROM, further sync with legacy-hal and sam's
hal and a few more minor fixes.

I think after this we are ready to implement WME on the driver
part. Anyone interested ?

Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>