MIPS: Fix debugfs_create_*'s error checking method for arch/mips/math-emu/

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

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>