ath9k: disable radio when all devices are marked idle

This uses the new configuration changes indicated up by
mac80211 when all interfaces are marked idle. We need to do
a little more work as we have our own set of virtual
wiphys within ath9k.

Only when all virtual wiphys are inactive do we allow an idle
state change for a wiphy to trigger disabling the radio.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>