net: dsa: mv88e6xxx: add port duplex setter

Similarly to port's link, add setter to force port's half duplex, full
duplex or let normal duplex detection occurs.

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