net/mlx5: qos: Store the eswitch in a mlx5_esw_rate_group

The rate groups are about to be moved out of eswitches, so store a
reference to the eswitch they belong to so things can still work
later.

This allows dropping the esw parameter from a couple of functions and
simplifying some of the code. Use this opportunity to make sure that
vport scheduling element commands are always sent to the group eswitch,
because that will be relevant for cross-esw scheduling. For now though,
the eswitches are not different.

There is no functionality change here.

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