net: ethernet: mediatek: fix logic unbalance between probe and remove

original mdio_cleanup is not in the symmetric place against where
mdio_init is, so relocate mdio_cleanup to the right one.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>