net: mdio: thunder: Fix a double free issue in the .remove function

'bus->mii_bus' have been allocated with 'devm_mdiobus_alloc_size()' in the
probe function. So it must not be freed explicitly or there will be a
double free.

Remove the incorrect 'mdiobus_free' in the remove function.

Fixes: 379d7ac7 ("phy: mdio-thunder: Add driver for Cavium Thunder SoC MDIO buses.")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>