mlx4_en: Fix double pci unmapping.

In cases of fragmented skb, with the data pointers being wrapped around
the TX buffer, the completion handling code would not forward the data
pointer and the firs fragment was unmapped several times, while others
were not unmapped at all.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>