net/fsl: remove irq assignment from xgmac_mdio

Which is wrong and not used, so no extra space needed by
mdiobus_alloc_size(), use mdiobus_alloc() instead.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>