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: 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>
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 56fb18d..70b3b44 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1529,7 +1529,7 @@
 	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;
 	}