net: hns3: refactor function hns3_get_vector_ring_chain()

Currently  hns3_get_vector_ring_chain() is a bit long. Refactor it by
extracting sub process to improve the readability.

Signed-off-by: Jie Wang <wangjie125@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>