net: dsa: mv88e6xxx: complete ATU state definitions

Marvell has different values for the state of a MAC address,
depending on its multicast bit. This patch completes the definitions
for these states.

At the same time, use 0 which is intuitive enough and simplifies the
code a bit, instead of the UC or MC unused value.

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