PCI: Fix NULL pointer dereference in of_pci_bus_find_domain_nr()

If the "parent" pointer passed to of_pci_bus_find_domain_nr() is NULL,
don't dereference it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>