I have had a Uno R3 for some time and have never been able to put it into DFU mode. I have updated the firmware over ISP, and now I want to use FLIP. I have loaded the UNO-dfu_usbserial_combined.hex file in arduino 1.6.5> hardware> arduino> avr> firmwares> atmegaxxu2 , and I can't put it into...