core,plugin-manager: fix warnings with -Wswitch-default

mm-plugin-manager.c: In function ‘plugin_supports_port_ready’:
  mm-plugin-manager.c:480:5: warning: switch missing default case [-Wswitch-default]
    480 |     switch (support_result) {
        |     ^~~~~~

src/mm-plugin-manager.c

@@ -495,6 +495,8 @@ plugin_supports_port_ready (MMPlugin     *plugin,
     case MM_PLUGIN_SUPPORTS_PORT_DEFER_UNTIL_SUGGESTED:
         port_context_defer_until_suggested (port_context, plugin);
         break;
+    default:
+        g_assert_not_reached ();
     }
 
     /* We received a full reference, to make sure the context was always