PCI: pci_is_root_bus helper

Introduce pci_is_root_bus helper function. This will help make code
more consistent, as well as prevent incorrect assumptions (such as
pci_bus->self == NULL on a root bus, which is not always true).

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