net: dsa: seville: duplicate vsc9959_mdio_bus_free

While we don't plan on making any changes to this function, currently
this is the only remaining dependency between felix and seville, after
the PCS has been refactored out into pcs-lynx.c.

Duplicate this function in seville to break the dependency completely.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>