bnx2x: fix TCP offload for tunneling ipv4 over ipv6

FW was initialized with data from wrong header, this caused TSO packets
have wrong IP csum.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>