Two tough years have taken a toll on Medtronic plc with a warning letter for its diabetes unit, a reorganization that will spin off its respiratory, patient care and renal groups and layoffs of an undisclosed number of employees that began last week. The announcement late Friday of the U.S. FDA approval of the Minimed 780G insulin pump system could be a turning point for the company that brings it back into the competition for the growing diabetes market.