net: dsa: mv88e6xxx: assign dynamic FDB to bridges

Give a new bridge a fresh FDB, assign it to its members, and restore a
fresh FDB to a port leaving a bridge.

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