Earlier versions of tg3 devices had a problem where the read DMA FIFO
could be overrun in certain edge conditions.  The fix was to limit the
number of rx BDs the hardware would fetch at a time.  For later devices
(5761, 5784 and later ASIC revs), there is a hardware fix that must be
enabled to fix the same problem.  This patch adds that hardware fix.

There is a gap in the ASIC revision lineage where neither fix is
applied.  This is intentional as these ASIC revisions are not afflicted
by the bug.
Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>