net/mlx5: fs, add unused destination type

When the caller doesn't pass a destination fs_core will create a unused
rule just so a context can be returned. This unused rule
is zeroed out and its type is 0 which can be mixed up with
MLX5_FLOW_DESTINATION_TYPE_VPORT.

Create a dedicated type to differentiate between the two
named MLX5_FLOW_DESTINATION_TYPE_NONE.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>