iwlwifi: set read/write permission for debugfs files

Set the correct Read/Write file permission for iwlwifi debugfs files
based on the functionality of the files

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>