net: dsa: mv88e6xxx: flush all addresses when adding a VLAN

When choosing an address database for a new VLAN, flush every entries,
not only the non-static ones.

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