powerpc/pseries: Checking for pdn->parent is redundant

The device tree root is never a pci bus, and will not have a
PCI_DN(pdn), so the check for PCI_DN added in
650f7b3b makes the check for pdn->parent
redundant and it can be removed.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>