net: dsa: mv88e6xxx: simulate Amethyst PHY model number

Amethyst internal PHYs also report empty model number in MII_PHYSID2.

Fill in switch product number, as is done for Topaz and Peridot.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>