bnx2x: fix tunneling CSUM calculation

Since commit c957d09ffda417f6c8e3d1f10e2b05228607d6d7
"bnx2x: Remove sparse and coccinelle warnings"
driver provided wrong partial csum for HW in tunneing
scenarios.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
CC: Pravin Shelar <pshelar@nicira.com>
CC: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Tested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>