mdio-sun4i: oops in error handling in probe

We could end up dereferencing an error pointer when we call
regulator_disable().

Fixes: 4bdcb1dd ('net: Add MDIO bus driver for the Allwinner EMAC')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>