ath9k: use SIMPLE_DEV_PM_OPS

ath9k does not provide any runtime pm callbacks, so support for
PM_RUNTIME is not needed and we could go to PM_SLEEP.
This also makes it possible to use SIMPLE_DEV_PM_OPS instead of
manually filling struct dev_pm_ops.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>