net: sparx5: Allow mdb entries to both CPU and ports

Allow mdb entries to be forwarded to CPU and be switched at the same
time. Only remove entry when no port and the CPU isn't part of the group
anymore.

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Acked-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>