net: hns3: add 8 BD limit for tx flow

A single transmit packet can span up to 8 descriptors according
to the HW limit. If a skb has more than 8 frags, driver uses
skb_copy to get a new skb which has less frags.

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>