OPP: fix error checking in opp_migrate_dentry()

Since commit ff9fb72b ("debugfs: return error values,
not NULL") changed return value of debugfs_rename() in
error cases from %NULL to %ERR_PTR(-ERROR), we should
also check error values instead of NULL.

Fixes: ff9fb72b

 ("debugfs: return error values, not NULL")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>