tg3: Adjust rx prod ring bd replenish thresholds

The oldest tg3 devices had large rx producer ring BD caches.  Back then,
it made sense to make the BD cache replenish threshold only a function
of the number of rx buffers posted by the driver.  Since then, the BD
cache sizes have shrunk to 25% of their original size and, in some
cases, the ring sizes have quadrupled in size.  Under such conditions,
static BD cache replenish thresholds no longer match the hardware
constraints.

This patch attempts to factor in the BD cache size into the bd cache
replenish strategy, taking the existing hardware bugs into account.

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