net/mlx4_core: A write memory barrier is sufficient in EQ ci update

Soften the memory barrier call of mb() by a sufficient wmb() in the
consumer index update of the event queues.

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