net: dsa: mv88e6xxx: use PORT_DEFAULT_VLAN

Minor, use the explicit PORT_DEFAULT_VLAN define instead of 0x07.

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