net: sxgbe: sxgbe_mdio_register() frees the bus

"err" is always zero at this point so we always unregister and free the
mdio_bus before returning success.  This seems like left over code and
I have deleted it.

Fixes: 1edb9ca6 ('net: sxgbe: add basic framework for Samsung 10Gb ethernet driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>