core: allow '2' when reading startup accept_ra value (bgo #691213)

Otherwise, priv->accept_ra_path would be NULL, which isn't very
useful and  makes nm_utils_do_sysctl() angry.  No reason we shouldn't
always create priv->accept_ra_path in the future though.

https://bugzilla.gnome.org/show_bug.cgi?id=691213

src/nm-device.c

@@ -346,7 +346,7 @@ update_accept_ra_save (NMDevice *self)
 	if (!nm_utils_get_proc_sys_net_value_with_bounds (priv->ip6_accept_ra_path,
 	                                                  ip_iface,
 	                                                  &priv->ip6_accept_ra_save,
-	                                                  0, 1)) {
+	                                                  0, 2)) {
 		g_free (priv->ip6_accept_ra_path);
 		priv->ip6_accept_ra_path = NULL;
 	}