[ath] fix "doing stuff before wakeup" warning; add comments for ACK/CTS handling...

[ath] fix "doing stuff before wakeup" warning; add comments for ACK/CTS handling during DFS/PASSIVE channels

* Although the hardware is awake, the power state handling doesn't think so.
  So just explicitly wake it up early in setup so ath_hal calls don't complain.

* We shouldn't be transmitting or ACKing frames during DFS CAC or on passive
  channels before we hear a beacon.  So, start laying down comments in the
  places where this work has to be done.

Note:

* The main bit missing from finishing this particular bit of work is a state
  call to transition a VAP from passive to non-passive when a beacon is heard.
  CAC is easy, it's an interface state.  So, I'll go and add a method to control
  that soon.