ath9k_hw: set pci_express capability true for AR9480

the AR_SREV register does not seems to indicate whether AR9480 is
pci_express capable or not though the other information like macVersion
etc can be obtained properly. this fix is essential as ASPM won't be intialized
and its related driver functionality ath9k_hw_configpcipowersave won't be
called

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>