tg3: 5719: Prevent tx data corruption

This patch enables a bit that prevents read DMA overflows and adjusts
the txmbuf margin from the hardware default.  The combination of these
modifications prevents a tx data corruption issue we were seeing on the
5719.

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