net: hns3: refactor mailbox response scheme between PF and VF

Currently, PF responds to VF depending on what mailbox it is
handling, it is a bit inflexible. The correct way is, PF should
check the mbx_need_resp field to decide whether gives response
to VF.

Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>