net: hns3: split function hns3_nic_get_stats64()

Function hns3_nic_get_stats64() is a bit too long. So add a
new function hns3_fetch_stats() to simplify code and improve
code readability.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>