powerpc/powernv: powernv_node_numa_domain() fix non-NUMA case

If NUMA is not enabled in the kernel config, or is disabled at boot, this
function should just return domain 0 regardless of what's in the device
tree.

Fixes a panic in iflib with NUMA disabled.

Reported by:	luporl

sys/powerpc/powernv/platform_powernv.c

@@ -517,6 +517,12 @@ powernv_node_numa_domain(platform_t platform, phandle_t node)
 	cell_t associativity[5];
 	int i, res;
 
+#ifndef NUMA
+	return (0);
+#endif
+	if (vm_ndomains == 1)
+		return (0);
+
 	res = OF_getencprop(node, "ibm,associativity",
 		associativity, sizeof(associativity));