iwlwifi: mvm: change RX sync notification to be an attribute and not a type

Currently the sync notification is a type of notification. However, it
is better fitted as an attribute of a notification, since there might
be another message in the payload (delba for instance) that should be
sent while control path is waiting for all queues to process.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>