net: dsa: bcm_sf2: Wire up MDB operations

Leverage the recently add b53_mdb_{add,del,prepare} functions since they
work as-is for bcm_sf2.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>