ath9k: merge common ->config_pci_powersave() checks

Move common checks into wrapper function. Since ASPM can be only enabled
on PCIe devices ->is_pciexpress check is unneeded.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>