mlxsw: reg: Extend mlxsw_reg_mpat_pack()

To support encapsulated SPAN, extend mlxsw_reg_mpat_pack() with a field
to set the SPAN type.

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