Trump is right.  If we go to them with requests to talk, nothing will happen.  With action, a total blockaide of all shipping into and out of NK, or continuing to shut down and freeze accounts of rich N Korean's, etc., will cause distress within the leadership of NK.  
When the pressure become great enough, they will come to us and then perhaps talks can ensue.
However, as with Iran, we had them on the ropes until Obama caved or before that North Vietnam, we had them close to surrender until Nixon caved.
Nothing will happen if we go to them.  When they come to us, perhaps progresss can be made.
Mattis is just saying the politically correct thing to say.