IB/qib: Use pci_is_root_bus() to check whether it is a root bus

Use pci_is_root_bus() instead of "if (bus->parent)" statement
for better readability.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@intel.com>

drivers/infiniband/hw/qib/qib_pcie.c

@@ -590,7 +590,7 @@ static int qib_tune_pcie_caps(struct qib_devdata *dd)
 
 	/* Find out supported and configured values for parent (root) */
 	parent = dd->pcidev->bus->self;
-	if (parent->bus->parent) {
+	if (!pci_is_root_bus(parent->bus)) {
 		qib_devinfo(dd->pcidev, "Parent not root\n");
 		goto bail;
 	}