iwlwifi: fix permissions on debugfs files

debugfs files are created with 644 permissions which gives everybody
read access. This presents a security issue if a user opens the file and
holds it open at the time the driver removes the file. At this point
invalid memory will be accessed.

Fix this by only allowing root to read debugfs files.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>