tg3: Turn off multiple DMA reads for 5717

The 5717's DMA read engine has a bug when initiating multiple DMA reads
across the PCIe bus.  This patch disables the feature.

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