net: hns3: fix for multiple unmapping DMA problem

When sending a big fragment using multiple buffer descriptor,
hns3 does one maping, but do multiple unmapping when tx is done,
which may cause unmapping problem.

To fix it, this patch makes sure the value of desc_cb.length of
the non-first bd is zero. If desc_cb.length is zero, we do not
unmap the buffer.

Fixes: 76ad4f0e

 ("net: hns3: Add support of HNS3 Ethernet Driver for hip08 SoC")
Signed-off-by: Fuyun Liang <liangfuyun1@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>