iwlwifi: mvm: update powersave correctly for D3

This fixes a long-standing bug - we haven't been able to check the
firmware image that was loaded for D3/not-D3 since the introduction
of the unified image...

Fix this by keeping a status flag for D3 instead of checking for
the firmware image that's loaded.

This reduces occurrences of checks for IWL_UCODE_WOWLAN to just the
code that actually loads the image or deals with it in other ways.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>