mlxsw: spectrum_qdisc: Index future FIFOs by band number

mlxsw used to hold an array of qdiscs indexed by the TC number. In the
previous patch, it was changed to allocate child qdiscs dynamically, and
they are now indexed by band number. Follow suit with the array of future
FIFOs.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>