powerpc/pci: cleanup on duplicate assignment

While creating the PCI root bus through function pci_create_root_bus()
of PCI core, it should have assigned the secondary bus number for the
newly created PCI root bus. Thus we needn't do the explicit assignment
for the secondary bus number again in pcibios_scan_phb().

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