net/mlx5e: Trust kernel regarding transport offset

After AF_PACKET is fixed to calculate the transport header offset
correctly, trust the value set by the kernel. If the offset wasn't set,
it means there is no transport header in the packet.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>