driver core: bus_add_driver should return an error if no bus

As pointed out by Dave Jones.

Cc: Dave Jones <davej@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/base/bus.c

@@ -603,7 +603,7 @@ int bus_add_driver(struct device_driver *drv)
 	int error = 0;
 
 	if (!bus)
-		return 0;
+		return -EINVAL;
 
 	pr_debug("bus %s: add driver %s\n", bus->name, drv->name);
 	error = kobject_set_name(&drv->kobj, "%s", drv->name);