iwlwifi: use new mac80211 SMPS

Instead of hard-coding the SM PS mode per hardware,
this makes iwlwifi support the new mac80211 API for
controlling the SM PS mode.

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