net/mlx5: qos: Refactor and document bw_share calculation

The previous function (esw_qos_calculate_group_min_rate_divider) had two
completely different modes of execution, depending on the 'group_level'
parameter. Split it into two separate functions:
- esw_qos_calculate_min_rate_divider - computes min across groups.
- esw_qos_calculate_group_min_rate_divider - computes min in a group.

Fold the divider calculation into the corresponding normalize functions
to avoid having the caller compute the corresponding divider.
Also rename the normalize functions to better indicate what level
they're operating on.
Finally, document everything so that this topic can more easily be
understood by future maintainers.

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