net/hns3: fix order of clearing imissed register in PF

net/hns3: fix order of clearing imissed register in PF

Clearing imissed registers in PF hardware depends on the
'drop_stats_mode' in struct hns3_hw. The variable is initialized after
the "hns3_get_configuration". But, in current code, the clearing
operation runs before the function.
So this patch fixes this order. In addition, this patch extracts a
public function to initialize and uninitialize statistics to improve the
maintainability of these codes.

Fixes: 3e9f3042d7c8 ("net/hns3: add imissed packet stats")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>