ath9k: check old power mode before clearing cycle counters

ath9k_ps_wakeup() clears the cycle counters after waking up the
hardware using ath9k_hw_setpower, however if power save is disabled,
then the counters will contain useful data, which then gets discarded.
Fix this by checking the old power mode before discarding any data.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>