LONDON, Sept 7( Reuters)-Nigerian state oil firm NNPC Ltd says a subsidiary of Italy's Eni did not obtain its consent prior to announcing a deal to sell onshore oil assets to local firm Oando PLC, a failure that could have breached terms of a joint operating agreement, according to a letter seen by Reuters. The letter casts doubt on the speed of the transaction, announced on Monday, and underscores the difficulty international oil majors have faced in their years-long efforts to sell onshore oil