net/mlx4_en: mlx4_en_netpoll() should schedule TX, not RX

I am not sure mlx4_en_netpoll() is doing anything useful right now.

mlx4 has different NAPI structures for RX and TX, and netpoll only wants
to drain TX queues.

Lets schedule NAPI polls on TX, not RX.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>