MIPS: Fix debugfs_create_*'s error checking method for mips/kernel/

debugfs_create_*() returns NULL on error.  Make its callers return -ENODEV
on error.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>