mlxsw: spectrum: Use port_module_max_width to compute base port index

Instead of using constant value, use port_module_max_width which is
aligned with the cluster size.

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