iwlwifi: enable power save setting upon config

This patch enables power save setting from config (iwconfig power)
The sysfs power_level interface is still preserved as it has
mac80211 power implementation is not yet rich enough.

Signed-off-by: Ester Kummer <ester.kummer@intel.com>
Reviewed-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>