net/mlx5e: Make the log friendly when decapsulation offload not supported
If we try to offload decapsulation actions to VFs hw, we get the log [1].
It's not friendly, because the kind of net device is null, and we don't
know what '0' means.
[1] "mlx5_core 0000:05:01.2 vf_0: decapsulation offload is not supported for net device (0)"