PCI: OF: Don't crash when bridge parent is NULL.

In pcibios_get_phb_of_node(), we will crash while booting if
bus->bridge->parent is NULL.

Check for this case and avoid dereferencing the NULL pointer.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>