powerpc/pci: Remove duplicate check in pcibios_fixup_bus()

pci_read_bridge_bases() already checks if the PCI bus is root
bus or not, so we needn't do same check in pcibios_fixup_bus()
and just remove it.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>