iwlwifi: set sm_ps_mode as part of cfg parameters

Setting "Spatial multiplexing Power Save" as part of
per device configuration parameter.

Report to uCode based on priv->conf setting, so driver can
have more control of how different devices should operate
in power save mode.

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>