PCI: check if bus has a proper bridge device before triggering SBR

For devices attached to the root bus, we can't trigger Secondary Bus
Reset because there is no bridge device associated with the bus. So
need to check bus->self again NULL first before using it.
Reviewed-by: NKenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: NYu Zhao <yu.zhao@intel.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/pci.c

@@ -2171,7 +2171,7 @@ static int pci_parent_bus_reset(struct pci_dev *dev, int probe)
 	u16 ctrl;
 	struct pci_dev *pdev;
 
-	if (dev->subordinate)
+	if (pci_is_root_bus(dev->bus) || dev->subordinate || !dev->bus->self)
 		return -ENOTTY;
 
 	list_for_each_entry(pdev, &dev->bus->devices, bus_list)