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

mm-iface-modem-time.c: In function ‘interface_disabling_step’:
  mm-iface-modem-time.c:481:5: error: switch missing default case [-Werror=switch-default]
    481 |     switch (ctx->step) {
        |     ^~~~~~
  ...

src/mm-iface-modem-time.c

@@ -520,6 +520,9 @@ interface_disabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -686,6 +689,9 @@ interface_enabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -858,6 +864,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();