net: phy: Fix mask so that we can identify Marvell 88E1518

The mask for the 88E1510 meant that the 88E1518 code would never be
used.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>