ath9k: Use pci_is_pcie()

Use function pci_is_pcie() instead of accessing struct member directly.

CC: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>