net: ethernet: mtk_eth_soc: fix resource leak in error path

In mtk_probe(), when mtk_ppe_init() or mtk_eth_offload_init() failed,
mtk_mdio_cleanup() isn't called. Fix it.

Fixes: ba37b7ca ("net: ethernet: mtk_eth_soc: add support for initializing the PPE")
Fixes: 502e84e2 ("net: ethernet: mtk_eth_soc: add flow offloading support")
Signed-off-by: Yan Cangang <nalanzeyu@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>