PCI: use pci_is_root_bus() in pci_get_interrupt_pin()

Use pci_is_root_bus() in pci_get_interrupt_pin() for checking if the
pci bus is root, for code consistency.
Reviewed-by: NAlex Chiang <achiang@hp.com>
Reviewed-by: NGrant Grundler <grundler@parisc-linux.org>
Signed-off-by: NKenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/pci.c

@@ -1529,7 +1529,7 @@ pci_get_interrupt_pin(struct pci_dev *dev, struct pci_dev **bridge)
 	if (!pin)
 		return -1;
 
-	while (dev->bus->parent) {
+	while (!pci_is_root_bus(dev->bus)) {
 		pin = pci_swizzle_interrupt_pin(dev, pin);
 		dev = dev->bus->self;
 	}