net: hns3: some variable modification

This patch does following things:
1. add the keyword const before some variables which won't be modified
   in functions.
2. changes some variables from signed to unsigned to avoid bitwise
   operation on signed variables.
3. adds or removes initialization of some variables.
4. defines a new structure to help parsing mailbox messages instead of
   using an array which is harder to get the meaning of each element.

Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>