iwlwifi: mvm: prepare the ground for more RSS notifications

We will need a new type of synchronization message going
through all the RSS queues. Prepare the ground for this.

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