tg3: Adjust BD replenish thresholds

The BD replenish thresholds for the 57765 and newer ASIC revs are a
little strict.  They were tuned for a mode that is currently unused.
This patch relaxes the thresholds so that they are set to values more
inline with the resources available.

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