net: dsa: slave: Handle switchdev host mdb add/del

Add code to handle switchdev host mdb add/del. Since DSA uses one of
the switch ports as a transport to the host, we just need to add an
MDB on this port.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>