net: mdio: mvusb: Fix an error handling path in mvusb_mdio_probe()

Should of_mdiobus_register() fail, a previous usb_get_dev() call should be
undone as in the .disconnect function.

Fixes: 04e37d92 ("net: phy: add marvell usb to mdio controller")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>