net/mlx5: E-Switch, Fail mlx5_esw_modify_vport_rate if qos disabled

Avoid calling mlx5_esw_modify_vport_rate() if qos is not enabled and
avoid unnecessary syndrome messages from firmware.

Fixes: fcb64c0f ("net/mlx5: E-Switch, add ingress rate support")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>