net/mlx5: Eswitch, Fix forwarding decision to uplink

Make sure to modify the rule for uplink forwarding only for the case
where destination vport number is MLX5_VPORT_UPLINK.

Fixes: 94db3317 ("net/mlx5: Support multiport eswitch mode")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>