ath9k_htc: Fix bug in handling CONF_IDLE

Disable the radio only when mac80211 indicates it,
through the IEEE80211_CONF_CHANGE_IDLE flag.
Not handling this properly will result in multiple
calls to radio_disable() even though the radio is
already idle.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>