net: hns3: modify return value in hns3_dbg_cmd_write

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

This patch modifies return value from 0 to -ENOSPC when buffer size
is greater than limit value, as in this case means the string buffer
is lllegal.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Reviewed-by: Zhong Zhaohui <zhongzhaohui@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>