ath9k_htc: Handle IDLE LED properly

Switch LED off/on when handling CONF_CHANGE_IDLE.
Not doing this would leave the radio LED on even
though the chip would be in full sleep mode.

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