If an error occurs, debugfs_create_file() will return ERR_PTR(-ERROR),
so use IS_ERR() to check it.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Signed-off-by: Thierry Reding <treding@nvidia.com>