Fix debugfs_create_*'s error checking method for wireless/rt2x00/

debugfs_create_*() 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>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>