iwlwifi: fix kernel crash when unregistering thermal zone

iwlwifi: fix kernel crash when unregistering thermal zone

A recent firmware change seems to have enabled thermal zones on the
iwlwifi driver. Unfortunately, my device fails when registering the
thermal zone. This doesn't stop the driver from attempting to unregister
the thermal zone at unload time, triggering a NULL pointer deference in
strlen() off the thermal_zone_device_unregister() path.

Don't unregister if name is NULL, for that case we failed registering.
Do the same for the cooling zone.

Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>