net/mlx4_en: Fix off-by-one in counters manipulation

This caused the en_stats_adder helper to accumulate a field which is
not related to the counter, fix that.

Fixes: a3333b35 ('net/mlx4_en: Moderate ethtool callback to show [..]')
Signed-off-by: NEran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_port.c

@@ -139,7 +139,7 @@ static unsigned long en_stats_adder(__be64 *start, __be64 *next, int num)
 	int i;
 	int offset = next - start;
 
-	for (i = 0; i <= num; i++) {
+	for (i = 0; i < num; i++) {
 		ret += be64_to_cpu(*curr);
 		curr += offset;
 	}