net: dsa: mv88e6xxx: use link-down-define instead of plain value

Using the define here makes the code more expressive.

Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>