PCI: fix wrong assumption in pci_read_bridge_bases

Current pci_read_bridge_bases() has an assumption that pci_bus->self
is NULL on the pci root bus (It checks pci_bus->self to see if the pci
bus is root bus). But is might not true on some platforms. We must
check pci_bus->parent instead.

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