Change the default method for device_quiesce() to return 0 instead of

EOPNOTSUPP.  The current behavior can mask real quiesce errors since
devclass_quiesce_driver() stops iterating over drivers as soon as it
gets an error (incluiding EOPNOTSUPP), but the caller it returns the
error to explicitly ignores EOPNOTSUPP.

Reviewed by:	imp

sys/kern/device_if.m

@@ -60,7 +60,7 @@ CODE {
 
 	static int null_quiesce(device_t dev)
 	{
-	    return EOPNOTSUPP;
+	    return 0;
 	}
 };