ath9k: Handle full sleep in ps_restore.

IDLE PS (Full Sleep) doesn't work when ifconfig up
is done during Idle unassociated state.
Fix this by restoring FULL SLEEP in ps_restore if CONF_IDLE
is set.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>