mlxsw: spectrum: Add lowest_shaper_bs to struct mlxsw_sp

Lower limit of burst size configuration is dependent on system type. Add a
datum to track the value. Initialize as appropriate in mlxsw_spX_init().

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