You need to modify the R8125 source code and compile it yourself. Since this is not easy, I recommend you to uninstall the R8125 plugin and use the built-in R8169 driver, probably it doesn't have ASPM settings. If the R8169 linux built-in driver also disables ASPM, then you have to find a solution yourself.
BTW: If you are modifying C states in BIOS, it might not only save energy from card but also CPU. 4 watts are too much for just one network card.