riscv: cacheinfo: Fix node reference leak in populate_cache_leaves

In the Linux kernel, the following vulnerability has been resolved: riscv: cacheinfo: Fix node reference leak in populate_cache_leaves Currently, the while loop drops the reference to prev in each iteration. If the loop terminates early due to a break, the final of_node_put(np) correctly drops the reference to the current node. However, if the loop terminates naturally because np == NULL, calling of_node_put(np) is a no-op. This leaves the last valid node stored in prev without its reference dropped, resulting in a node reference leak. Fix this by changing the final `of_node_put(np)` to `of_node_put(prev)`.