net/mlx5e: Fix traffic duplication in ethtool steering

Before this patch, when adding multiple ethtool steering rules with
identical classification, the driver used to append the new destination
to the already existing hw rule, which caused the hw to forward the
traffic to all destinations (rx queues).

Here we avoid this by setting the "no append" mlx5 fs core flag when
adding a new ethtool rule.

Fixes: 6dc6071c ("net/mlx5e: Add ethtool flow steering support")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>