mlxsw: Introduce mlxsw_reg_spms_vid_pack helper and use it

Introduce separate helper for packing SPMS VIDs, as it can be used for
multiple VIDs and not only for one as previous SPMS pack function
provided.

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