ath9k: fix going to full-sleep on PS idle

The check for PS_WAIT_FOR_TX_ACK was inverted, the hardware should only go
to full sleep if no tx is pending.

Reported-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>