bnx2x: fix UDP csum offload

Fixed packets parameters for FW in UDP checksum offload flow.

Do not dereference TCP headers on non TCP frames.
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>

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