PCIe port bus: use pci_pcie_cap()

Use pci_pcie_cap() instead of pci_find_capability() to get PCIe capability
offset in PCI Express Port Bus driver. This avoids unnecessary serarch
in PCI configuration space.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>