net/mlx5: E-Switch, fix syndrome (0x678139) when turn on vepa

Make sure the struct mlx5_flow_destination is zero before
filling in the field.

Fixes: 8da202b2

 ("net/mlx5: E-Switch, Add support for VEPA in legacy mode.")
Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>