RDMA/mlx5: Implement query_port_speed callback

RDMA/mlx5: Implement query_port_speed callback

Implement the query_port_speed callback for mlx5 driver to support
querying effective port bandwidth.

For LAG configurations, query the aggregated speed from the LAG layer
or from the modified vport max_tx_speed.

Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>