PCI: introduce pci_pcie_cap()

Introduce pci_pcie_cap() API that returns saved PCIe capability offset
(currently it is saved in 'pcie_cap' field in the struct PCI dev).
Using pci_pcie_cap() instead of pci_find_capability() avoids
unnecessary search in PCI configuration space.

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