phy/marvell: rename 88ec048 to 88e1318s and fix mscr1 addr

The marvell 88ec048's official part number is 88e1318s.  This patch renames
definitions in the driver to reflect this.

In addition, a minor bug fix has been added to write back the MSCR1 register
value properly.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>