mdio-sun4i: Fix a memory leak

If the probing of the regulator is deferred, the memory allocated by
'mdiobus_alloc_size()' will be leaking.
It should be freed before the next call to 'sun4i_mdio_probe()' which will
reallocate it.

Fixes: 4bdcb1dd

 ("net: Add MDIO bus driver for the Allwinner EMAC")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>