tg3: Improve 5785 PCIe performance

This patch improves 5785 performance by allowing the write DMA engine to
request larger DMA burst sizes than it otherwise would.

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