mlxsw: spectrum_span: Replace zero-length array with flexible-array member

In a similar fashion to commit e99f8e7f ("mlxsw: Replace zero-length
array with flexible-array member"), use a flexible-array member to get a
compiler warning in case the flexible array does not occur last in the
structure.

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