net: hns3: limit bd numbers when getting dfx regs.

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

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

When getting dfx regs, the bd numbers from the firmware may be
very big which is not expected, and cause the buffer size
overflow. This patch limit the max bd numbers to 64 to fix the
problem.

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