avoid bad register access in eephy_reset for Marvell Alaska 88E1512

avoid bad register access in eephy_reset for Marvell Alaska 88E1512

The register write before this change had no visible run-time effect
on 88E1512 under normal conditions but it tried to update a reserved
bit and might prevent the standard behaviour of breaking the link on
insufficient Inter-Packet Gaps.

Cross-checked with datasheets for supported variants. Extended special
control register 20 exists only in E1111 but not in E151x, E1545x, or
E3016. It seems that the register was dropped in E15xx and subsequent
designs.

Originally from stsp@

ok claudio@ stsp@