tg3: Enable mult rd DMA engine on 5719

The multiple DMA read engine bugs have been fixed on the 5719.  This
patch reenables support for this feature.

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