PCI: use pci_is_root_bus() in pci_common_swizzle()

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

drivers/pci/pci.c

@@ -1549,7 +1549,7 @@ u8 pci_common_swizzle(struct pci_dev *dev, u8 *pinp)
 {
 	u8 pin = *pinp;
 
-	while (dev->bus->parent) {
+	while (!pci_is_root_bus(dev->bus)) {
 		pin = pci_swizzle_interrupt_pin(dev, pin);
 		dev = dev->bus->self;
 	}