net: hns3: add check to number of buffer descriptors

This patch adds check to number of bds before we allocate memory for
them. If we get an invalid bd num in some cases, it will cause a memory
overflow.

Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>