driver model: make sure device has driver when we add it to the class.

There is a chance devclass_add_device() could get passed a device with no driver, so
we need to check that.

drivers/base/class.c

@@ -53,8 +53,11 @@ static void unenum_device(struct device_class * cls, struct device * dev)
 
 int devclass_add_device(struct device * dev)
 {
-	struct device_class * cls = dev->driver->devclass;
+	struct device_class * cls;
 	int error = 0;
+
+	if (dev->driver) {
+		cls = dev->driver->devclass;
 		if (cls) {
 			pr_debug("adding device '%s' to class '%s'\n",
 				 dev->name,cls->name);
@@ -65,6 +68,7 @@ int devclass_add_device(struct device * dev)
 				interface_add(cls,dev);
 			}
 		}
+	}
 	return error;
 }