net/mlx5: Fix mlx5_add_flow_rules call with correct num of dests

When adding ethtool steering rule with action DISCARD we wrongly
pass a NULL dest with dest_num 1 to mlx5_add_flow_rules().
What this error seems to have caused is sending VPORT 0
(MLX5_FLOW_DESTINATION_TYPE_VPORT) as the fte dest instead of no dests.
We have fte action correctly set to DROP so it might been ignored
anyways.

To reproduce use:
 # sudo ethtool --config-nfc <dev> flow-type ether \
   dst aa:bb:cc:dd:ee:ff action -1

Fixes: 74491de9

 ("net/mlx5: Add multi dest support")
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>