mlxsw: Remove multicast ID configuration

With respect to a firmware change, the Switch Multicast ID (SMID)
register is no longer needed, so the related configuration code can be
removed.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>