mlxsw: spectrum_span: Fix initialization of struct mlxsw_sp_span_parms

Since the first element of struct mlxsw_sp_span_parms is a pointer,
to zero-initialize this structure the correct notation is not = {0}, but
rather = {NULL}, as reported by sparse.

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