iwlwifi: mvm: Sync FW API time event notification change

The firmware API changed to differentiate between event and
fragment start/end. Change the time-event handling accordingly.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>