gianfar: incorrect TCP checksum when timestamps are enabled

[Alex Dubov]

Greetings.

I'm working on MPC8548 based board (I started with 2.6.37-rc7, now upgraded
to post release 2.6.37 git head).

It so appears, that when tcp timestamps are enabled (default setting)
TCP checksums of outgoing SYN packets are calculated incorrectly. This
causes remote machines to reject such packets, effectively preventing any
useful network use. The problem is reproducible every time.

Other protocols appear to work reliably (UDP/DHCP do work).

If tcp timestamps are disabled, checksums are calculated correctly.

Netdev guys tend to think that the problem is with hardware checksum
offloading, the theory I'm going to test now.