powerpc/pci: Remove duplicate logic

Since the logic to reset PCI secondary bus by PCI config register
PCI_BRIDGE_CTL_BUS_RESET is included in pci_reset_secondary_bus(), we
needn't implement another one.

Remove the duplicate implementation and call pci_reset_secondary_bus().

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>