Fix debugfs_create_bool's error checking method for wireless/iwlwifi/

debugfs_create_bool() 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: John W. Linville <linville@tuxdriver.com>