net: dsa: mv88e6xxx: add port link setter

Most of the chips will have a port register control bits to force the
port's link up, down, or let normal link detection occurs.

Implement such operation to use it later when setting duplex, etc.

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