net: dsa: mv88e6xxx: enable SA learning on DSA ports

In multi-chip systems, DSA Tag ports must learn SA addresses in order to
correctly switch frames between interconnected chips.

This fixes cross-chip hardware bridging in a VLAN filtering aware
system, because a bridge group gets implemented as an hardware 802.1Q
VLAN and thus DSA and user ports share the same FDB.

Fixes: 4c7ea3c0 ("net: dsa: mv88e6xxx: disable SA learning for DSA and CPU ports")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>