net/mlx5: fs, Command to control TX flow table root

Introduce an API to set/unset the TX flow table root for a device.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>