tg3: Fix stats for 5704 and later devices

Commit 4d958473, entitled
"tg3: Workaround rx_discards stat bug" modified the hardware statistics
data structure.  The modification shifted the statistics so that the
labels no longer corresponded to the counter values.  This patch fixes
the problem by utilizing reserved space for the new counters.

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