mlx4: do not use priv->stats_lock in mlx4_en_auto_moderation()

Per RX ring packets/bytes counters are not protected by global
priv->stats_lock.

Better not confuse the reader, and use READ_ONCE() to show we read
these counters without surrounding synchronization.

Interrupt moderation is best effort, and we do not really care of
ultra precise counters.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>