pciehp: use pci_pcie_cap()

Use pci_pcie_cap() instead of pci_find_capability() to get PCIe capability
offset in pciehp driver. This avoids unnecessary search in PCI
configuration space. This patch also removes 'cap_base' field in
struct controller, that was used to hold PCIe capability offset by
pciehp itself.

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