Add a SLEEP->SCAN transition.  Without this a disassociate or scan
request during SLEEP results in a hang.

Whilst I'm here, add in some disabled code that will transition to RUN
if there's multicast traffic.  It's not needed for Atheros hardware but
it may be for other hardware.

Tested:

* AR5416, STA mode (powersave)
* AR5212, STA mode (powersave)