port-qmi: fix warnings with -Wswitch-default

mm-port-qmi.c: In function ‘port_open_step’:
  mm-port-qmi.c:350:5: error: switch missing default case [-Werror=switch-default]
    350 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-port-qmi.c

@@ -590,6 +590,9 @@ port_open_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }