mlxsw: spectrum: Mark unused ports using NULL

When splitting and unsplitting we'll destroy usable ports on the fly, so
mark them using a NULL pointer to indicate that their local port number
is free and can be re-used.

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>