PCI: use pci_is_root_bus() in pci_find_upstream_pcie_bridge()

Use pci_is_root_bus() in pci_find_upstream_pcie_bridge() to check 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/search.c

@@ -29,7 +29,7 @@ pci_find_upstream_pcie_bridge(struct pci_dev *pdev)
 	if (pdev->is_pcie)
 		return NULL;
 	while (1) {
-		if (!pdev->bus->parent)
+		if (pci_is_root_bus(pdev->bus))
 			break;
 		pdev = pdev->bus->self;
 		/* a p2p bridge */