powerpc/fman/memac: use default MDIO_HOLD value

Current driver uses a Maximum value for MDIO_HOLD when doing 10G MDIO
access, this is due to an errata A-006260 on T4 rev1.0 which is fixed
on rev2.0, so remove the maximum value to use the default value for rev2.0.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>