OPP: fix error checking in opp_migrate_dentry()

[ Upstream commit eca4c0eea53432ec4b711b2a8ad282cbad231b4f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>