net: hns3: refactor for hns3_fill_desc() function

Factor out hns3_fill_desc() so that it can be reused in the
tx bounce supporting.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>