lan78xx: Remove hardware-specific header update

Remove hardware-specific header length adjustment as it is no longer
required. It also breaks generic receive offload (GRO) processing of
received TCP frames that results in a TCP ACK being sent for each
received frame.

Signed-off-by: John Efstathiades <john.efstathiades@pebblebay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>