net: hns3: refactor hns3_get_new_int_gl function

This patch adds a new hns3_get_new_flow_lvl function to calculate
the packet flow level, which is used to decide the interrupt
coalescence parameter, in order to make the flow level calculation
code more readable and make the future calculation ajdustment easier.

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>