net/mlx5: Initialize destination_flow struct to 0

This is needed in order to enlarge it with more members that will get
value of 0 when not set.

Signed-off-by: Rabie Loulou <rabiel@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>