net/mlx5: qos: Always create group0

All vports not explicitly members of a group with QoS enabled are part
of the internal esw group0, except when the hw reports that groups
aren't supported (log_esw_max_sched_depth == 0). This creates corner
cases in the code, which has to make sure that this case is supported.
Additionally, the groups are about to be moved out of eswitches, and
group0 being NULL creates additional complications there.

This patch makes sure to always create group0, even if max sched depth
is 0. In that case, a software-only group0 is created referencing the
root TSAR. Vports can point to this group when their QoS is enabled and
they'll be attached to the root TSAR directly. This eliminates corner
cases in the code by offering the guarantee that if qos is enabled,
vport->qos.group is non-NULL.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>