parisc: fix wrong assumption about bus->self

Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com> posted a patch series
to linux-pci to fix a wrong assumption about pci_bus->self==NULL for
all PCI host bus controllers. While PARISC platforms to not behave
this way, I prefer to have the code consistent across architectures.
The following patch replaces pci_bus->self with pci_bus->parent when
used as a test to check for "root bus controller".

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>