ci: exclude spiffs submodule from flake8 checks

.flake8

@@ -149,6 +149,7 @@ exclude =
         components/openthread/openthread,
         components/tinyusb,
         components/unity/unity,
+        components/spiffs/spiffs,
         examples/build_system/cmake/import_lib/main/lib/tinyxml2,
         examples/peripherals/secure_element/atecc608_ecdsa/components/esp-cryptoauthlib,
     # autogenerated scripts