mlxsw: spectrum_switchdev: Add a ports bitmap to the mid db

Add a bitmap of ports to the mid struct to hold the ports that are
registered to this mid.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>