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: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>