iwlwifi: show current power save status reported by uCode

Power save request is sent from driver to uCode, but there is no
indication from uCode about the current device power save state.

Reading GP_CNTRL register bit 25:24 to show the current power save
status

00: no power save
01: MAC power down
10: PHY power down
11: Error

The uCode could switch in and out of power save mode in the order of
once per 100-300 ms in many cases. The reading here should just be used for
reference on the current uCode power save status. Do not confuse this
reading with the PowerSave set by driver and mac80211.

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>