net: dsa: Remove support for MDB dump from DSA's drivers

This is done as a preparation before removing support for MDB dump from
DSA core. The MDBs are synced with the bridge and thus there is no
need for special dump operation support.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>