iwlwifi: fix legacy thermal throttling power index

For legacy thermal throttling, set the new Thermal Throttling
state and change power index when thermal throttling manager detects
temperature changed. The current implementation sets the state to the
previous Thermal Throttling state, which causes system to enter wrong
power index. The worse case, it will trying to set the
lower power index when device reach critical temperature, it will cuase
issue for both system and the device.

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>