iwlwifi: add Greenfield support for 11n

Add "Greenfield" support for all devices except 4965 and
3945. "Greenfield" is part of 11n features to improve HT performance.

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>