ath9k: Set HW state properly

This patch fixes a bug in ath9k_stop() where the HW
was not put into FULL_SLEEP state. Not doing so will
cause issues in suspend-resume and the HW will not respond
to chip resets.

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