Arduino Uno R1 I soldered resistor but I cannot DFU program atmega8u2
According to https://gebloggendings.wordpress.com/category/elektronik/arduino/ it is sometimes not to assume that the atmega8u2 is always the correct value in my case was the at90usb82 resulting into using the following command sequence:

sudo dfu-programmer at90usb82 erase
# Flashing the arduino provided .hex
sudo dfu-programmer at90usb82 /usr/share/arduino/hardware/arduino/firmwares/atmegaxxu2/arduino-usbserial/Arduino-usbserial-uno.hex
# reset the board 
sudo dfu-programmer at90usb82 reset


Further information you can find into:


https://forum.arduino.cc/index.php?topic=53443.0 
https://forum.arduino.cc/index.php?topic=542854.0