tg3: Apply rx_discards fix to 5719/5720

Commit 4d958473, entitled
"tg3: Workaround rx_discards stat bug", was intended to be applied to
the 5717, 5718, 5719_A0, and 5720 A0 chip revisions.  The implementation
missed the latter two when applying the fix in a critical area.  This
patch fixes the problem.

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