net/mlx5: fix tunnel offload validation

net/mlx5: fix tunnel offload validation

Tunnel offload API allows the application to restore packet to
its original form if the chain of flows is missed after DECAP action.

MLX5 PMD provides tunnel offload support only if DV API was enabled.

The patch verifies DV availability before processing with
tunnel offload tasks.

Fixes: 4ec6360de37d ("net/mlx5: implement tunnel offload")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>