net: hibmcge: fix the division by zero issue

In the Linux kernel, the following vulnerability has been resolved: net: hibmcge: fix the division by zero issue When the network port is down, the queue is released, and ring->len is 0. In debugfs, hbg_get_queue_used_num() will be called, which may lead to a division by zero issue. This patch adds a check, if ring->len is 0, hbg_get_queue_used_num() directly returns 0.