PCI: Use pci_is_root_bus() to check for root bus

In __pci_bus_size_bridges() we check whether a bus is a root bus by testing
bus->self.  As indicated by commit 79af72d7 ("PCI: pci_is_root_bus
helper"), bus->self == NULL is not a proper way to check for a root bus.
One issue is that "virtual" buses added for SR-IOV (via virtfn_add_bus())
have bus->self == NULL but are not root buses.

This patch changes it to pci_is_root_bus() to check whether it is a root
bus.

[bhelgaas: changelog]
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>