powerpc/pci: Use for_each_pci_bridge() helper

Use for_each_pci_bridge() helper to make the code slightly cleaner.  No
functional change intended.

Requires: 24a0c654 ("PCI: Add for_each_pci_bridge() helper")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>