silent sms: Fix the silent SMS feature once again

It got broken in the refactoring of 20474ad02983861cb6a9542916eaeb78a6615604.
diff --git a/openbsc/src/vty_interface_layer3.c b/openbsc/src/vty_interface_layer3.c
index 56b4966..526ae9e 100644
--- a/openbsc/src/vty_interface_layer3.c
+++ b/openbsc/src/vty_interface_layer3.c
@@ -248,7 +248,7 @@
 	}
 
 	str = argv_concat(argv, argc, 2);
-	rc = _send_sms_str(subscr, str, 0);
+	rc = _send_sms_str(subscr, str, 64);
 	talloc_free(str);
 
 	subscr_put(subscr);