iwlwifi: mvm: fail temp test enabling if the ucode is not loaded

If the ucode is not loaded, don't allow the temperature test to be
started, but allow it to be changed or stopped if already running.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>