net: hns3: refactor BD filling for l2l3l4 info

This patch separates the inner and outer l2l3l4 len handling in
hns3_set_l2l3l4_len, this is a preparation to combine the l2l3l4
len and checksum handling for inner and outer header.

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