tg3: Disable new DMA engine for 57766

A bug was found in the new DMA engine for the 57766.  This patch
disables it, which causes the device to fallback to the old DMA engine.

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