net: dsa: mv88e6xxx: unlock DSA and CPU ports

Locking a port generates an hardware interrupt when a new SA address is
received. This enables CPU directed learning, which is needed for 802.1X
MAC authentication.

To disable automatic learning on a port, the only configuration needed
is to set its Port Association Vector to all zero.

Clear PAV when SA learning should be disabled instead of locking a port.

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>