net/mlx4_en: use napi_complete_done() in TX completion

In order to benefit from the new napi_defer_hard_irqs feature,
we need to use napi_complete_done() variant in this driver.

RX path is already using it, this patch implements TX completion side.

mlx4_en_process_tx_cq() now returns the amount of retired packets,
instead of a boolean, so that mlx4_en_poll_tx_cq() can pass
this value to napi_complete_done().

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