net/mlx5e: Remove the wrong assumption about transport offset

skb_transport_offset() == 0 is not a special value. The only special
value is when skb->transport_header is ~0U, and it's checked by
skb_transport_header_was_set().

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