mlxsw: spectrum_qdisc: Offload root TBF as port shaper

The Spectrum ASIC allows configuration of maximum shaper on all levels of
the scheduling hierarchy: TCs, subgroups, groups and also ports. Currently,
TBF always configures a subgroup. But a user could reasonably express the
intent to configure port shaper by putting TBF to a root position, around
ETS / PRIO. Accept this usage and offload appropriately.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>