iwlwifi: add fall through comment

The fall-through to the MVM case is intended as we have to do
*something* to continue, and can't easily clean up. So we'll
just fail in mvm later, if this does happen.

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