[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.