nexus: Consistently return a pointer in failure paths

No functional change intended.

MFC after:	1 week

sys/arm/arm/nexus.c

@@ -256,7 +256,7 @@ nexus_alloc_resource(device_t bus, device_t child, int type, int *rid,
 	if (needactivate) {
 		if (bus_activate_resource(child, type, *rid, rv)) {
 			rman_release_resource(rv);
-			return (0);
+			return (NULL);
 		}
 	}
 

sys/x86/x86/nexus.c

@@ -388,13 +388,13 @@ nexus_alloc_resource(device_t bus, device_t child, int type, int *rid,
 
 	rv = rman_reserve_resource(rm, start, end, count, flags, child);
 	if (rv == NULL)
-		return (0);
+		return (NULL);
 	rman_set_rid(rv, *rid);
 
 	if (needactivate) {
 		if (bus_activate_resource(child, type, *rid, rv)) {
 			rman_release_resource(rv);
-			return (0);
+			return (NULL);
 		}
 	}