net/mlx5: Avoid configuring eswitch QoS if not supported

Check if QoS is enabled for the eswitch before attempting to configure
QoS parameters and emit a netlink error if not supported.

Introduce an API to check if QoS is supported for the eswitch.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>