net: dsa: mv88e6xxx: Workaround missing PHY ID on mv88e6390

The internal PHYs of the mv88e6390 do not have a model ID. Trap any
calls to the ID register, and if it is zero, return the ID for the
mv88e6390. The Marvell PHY driver can then bind to this ID.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>