net: dsa: mv88e6171: add hardware 802.1Q support

The Marvell 88E6171 switch is in the 88E6351 family, which supports
802.1Q, thus add support from the generic mv88e6xxx functions.

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>