tg3: Workaround rx_discards stat bug

The 5717, 5718, 5719 A0, and 5720 A0 has a bug where the rx_discards
statistic counter will increment when dropping unwanted multicast
frames.  This patch works around the problem by attempting to
recreate the data using other means.  The resulting value will not be
accurate, but it can still serve as a problem indicator.

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