amd-xgbe: unwind on error in xgbe_mdio_register()

There is a typo here so we return directly instead of unwinding.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>