net: dsa: mv88e6xxx: disable SA learning for DSA and CPU ports

Frames with DSA headers passing to/from the CPU were taking place in the
MAC learning on these ports, resulting in incorrect ATU entries. Disable
learning on these ports.

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