net: hns3: clean up for vlan handling in hns3_fill_desc_vtags

This patch refactors the hns3_fill_desc_vtags function
by avoiding passing too many parameters, reducing indent
level and some other clean up.

This patch also adds the hns3_fill_skb_desc function to
fill the first desc of a skb.

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