iface-modem-signal: fix warnings with -Wswitch-default

mm-iface-modem-signal.c: In function ‘interface_initialization_step’:
  mm-iface-modem-signal.c:436:5: error: switch missing default case [-Werror=switch-default]
    436 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-iface-modem-signal.c

@@ -501,6 +501,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();