gianfar:don't add FCB length to hard_header_len

FCB(Frame Control Block) isn't the part of netdev hard header.
Add FCB to hard_header_len will make GRO fail at MAC comparision stage.

Signed-off-by: Jiajun Wu <b06378@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>