tg3: Fix IPv6 TCP problems for 5719

tg3: Fix IPv6 TCP problems for 5719

Commit 4d163b75e979833979cc401ae433cb1d7743d57e, entitled
"tg3: Fix 5719 A0 tx completion bug" turned off TSO to fix a hardware
bug.  In doing so, it accidentally turned off all IPv6 TCP checksum
offloading too.  This patch fixes the problem by reenabling the hardware
bit that control both features.  The TSO capability is still not exposed
to the kernel.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>