Fix error value returned by ofw_bus_gen_get_node().

By definition ofw_bus_get_node() should  consistently return -1 when there
is no associated OF node.

MFC after:	4 weeks
Discussed with:	nwhitehorn
Analyzed in: 	https://reviews.freebsd.org/D30761

sys/dev/ofw/ofw_bus_subr.c

@@ -144,7 +144,7 @@ ofw_bus_gen_get_node(device_t bus, device_t dev)
 
 	obd = OFW_BUS_GET_DEVINFO(bus, dev);
 	if (obd == NULL)
-		return (0);
+		return ((phandle_t)-1);
 	return (obd->obd_node);
 }