iwlwifi: use S_IRUGO and S_IWUSR in module parameters

Instead of hardcode module parameter's permissions, use pre-defined.

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>