net_dma: remove duplicate assignment in dma_skb_copy_datagram_iovec

No need to compute copy twice in the frags loop in
dma_skb_copy_datagram_iovec().

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>