ath9k: fix a chip wakeup related crash in ath9k_start

When the chip is still asleep when ath9k_start is called,
ath9k_hw_configpcipowersave can trigger a data bus error.

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