PCI: use pci_is_root_bus() in acpi_find_root_bridge_handle()

Use pci_is_root_bus() in acpi_find_root_bridge_handle() to check 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>

include/linux/pci-acpi.h

@@ -15,7 +15,7 @@ static inline acpi_handle acpi_find_root_bridge_handle(struct pci_dev *pdev)
 {
 	struct pci_bus *pbus = pdev->bus;
 	/* Find a PCI root bus */
-	while (pbus->parent)
+	while (!pci_is_root_bus(pbus))
 		pbus = pbus->parent;
 	return acpi_get_pci_rootbridge_handle(pci_domain_nr(pbus),
 					      pbus->number);