iwlwifi: rename ps_mode to sm_ps

This patch renames iwl_priv.ps_mode for clearer
iwl_priv.current_ht_config.sm_ps (spatial multiplexing power save).

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-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>