phy: mdio-octeon: Use devm_mdiobus_alloc_size()

Rather than use devm_kzalloc(), use the mdio helper function.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>