net: hns3: bd_num from fireware should not be zero

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

----------------------------

The bd_num query from fireware maybe zero, use zero to alloc
buffer may cause override problem.

Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: li yongxin <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>