debugfs_create_dir() returns NULL if an error occurs, returns -ENODEV
when debugfs is not enabled in the kernel.
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>