net: hns3: minor optimization for datapath

This patch adds a likely case for hns3_fill_desc and
limits the local variables' scope as much as possible,
also avoid div operation when the tqp_vector->num_tqps
is one.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
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>