net: ethernet: mtk_eth_soc: fix possible memory leak in mtk_probe()

If mtk_wed_add_hw() has been called, mtk_wed_exit() needs be called
in error path or removing module to free the memory allocated in
mtk_wed_add_hw().

Fixes: 804775df ("net: ethernet: mtk_eth_soc: add support for Wireless Ethernet Dispatch (WED)")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>