net/hns3: fix mailbox communication with HW

net/hns3: fix mailbox communication with HW

Mailbox is the communication mechanism between SW and HW. There exist
two approaches for SW to recognize mailbox message from HW. One way is
using match_id, the other is to compare the message code. The two
approaches are independent and used in different scenarios.

But for the second approach, "next_to_use" should be updated and written
to HW register. If it not done, HW do not know the position SW steps,
then, the communication between SW and HW will turn to be failed.

Fixes: dbbbad23e380 ("net/hns3: fix VF handling LSC event in secondary process")
Cc: stable@dpdk.org
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>