net/mlx5: Refactor QoS group scheduling element creation

Introduce `esw_qos_create_group_sched_elem` to handle the creation of
group scheduling elements for E-Switch QoS, Transmit Scheduling
Arbiter (TSAR).

This reduces duplication and simplifies code for TSAR setup.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>