Using $tempnode also works:
ACTION=="add", SUBSYSTEM=="usb_device", ATTRS{idVendor}=="0763", ATTRS{idProduct}=="1031", ATTRS{bcdDevice}=="0121", RUN+="/sbin/fxload -s /lib/firmware/maudio/MidiSportLoader.ihx -I /lib/firmware/maudio/MidiSport8x8-2.21.ihx -D $tempnode"
Using $tempnode also works:
ACTION=="add", SUBSYSTEM= ="usb_device" , ATTRS{idVendor} =="0763" , ATTRS{idProduct }=="1031" , ATTRS{bcdDevice }=="0121" , RUN+="/sbin/fxload -s /lib/firmware/ maudio/ MidiSportLoader .ihx -I /lib/firmware/ maudio/ MidiSport8x8- 2.21.ihx -D $tempnode"