ath9k: optimize ath9k_ps_restore

ath_hw_cycle_counters_update only needs to be called if the power state
changes. Most of the time this does not happen, even when ps_usecount
goes down to 0.

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