iwlwifi: add SM PS support for 6x50 series

Spatial Multiplexing Power Save was disabled to achieve better
throughput while in power save mode by activating all the rx chains all the time.
By doing so, the device power consumption is high.

Enable static/dynamic spatial multiplexing power save if device support
it, which can lower the power consumption without impact throughput.

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>