tg3: Fix 5719 A0 tx completion bug

The 5719 A0 has a bug that manifests itself as if the chipset were
reordering memory writes.  The best known way to solve this problem is
to turn off LSO and jumbo frames.

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