wlan/atheros - Synchronize sleep state code from FreeBSD

wlan/atheros - Synchronize sleep state code from FreeBSD

* Synchronize Adrian Chadd's sleep state code and wlan updates.

* With this commit if the wlan or ath interface is in a 'down' state,
  it will use full sleep mode and save power.

* We get all of ATH through today but the 802_11 changes are a bit
  too substantial so this commit only brings in the sleep state code
  through today (3-May-2014).

* There is a bunch of other 802_11 work that needs to be brought in
  but it's like 20,000 lines of patches so... not today.

* Verified operational on Acer C720 chromebook.  full-sleep mode saves
  0.3-0.4W.  Network sleep mode code is also verified to be operational
  but does not appear to save any power at the moment.

  Also verified: beacons are working properly for wakeups on packet
  reception.