ath9k: restore PS mode, before we put the chip into FULL SLEEP state.

We want to put the chip into FULL SLEEP state, when we are disabling the
radio, but the the current code always change it to AWAKE/NETWORK SLEEP.

Changes-licensed-under: ISC
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>