cpu/stm32/periph/eth: Fix transmission bug
cpu/stm32/periph/eth: Fix transmission bug

Using the TER bit in the TX descriptors when only using a single descriptor for
sending triggered a hardware bug. Thus, stop using the TER bit and store the
currently active TX descriptor in RAM instead.

kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT