net: dsa: lantiq_gswip: remove unnecessary platform_set_drvdata()

Remove unnecessary platform_set_drvdata() in ->remove(), the driver_data
will be set to NULL in device_unbind_cleanup() after calling ->remove().

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>