mlx4: use napi_schedule_irqoff()

mlx4_en_rx_irq() and mlx4_en_tx_irq() run from hard interrupt context.

They can use napi_schedule_irqoff() instead of napi_schedule()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-By: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>