mlx4: dont orphan skbs in mlx4_en_xmit()

After commit e22979d9 (mlx4_en: Moving to Interrupts for TX
completions) we no longer need to orphan skbs in mlx4_en_xmit()
since skb wont stay a long time in TX ring before their release.

Orphaning skbs in ndo_start_xmit() should be avoided as much as
possible, since it breaks TCP Small Queue or other flow control
mechanisms (per socket limits)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yevgeny Petrilin <yevgenyp@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>