net: dsa: mv88e6xxx: implement DSA port fast ageing

Now that the DSA layer handles port fast ageing on correct STP change,
simplify _mv88e6xxx_port_state and implement mv88e6xxx_port_fast_age.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>