cpu/periph_eth: Fix kconfig model
cpu/periph_eth: Fix kconfig model

This probably can be done better as the periph_eth should not be
only part of the stm32 as the sam0 also uses it.

RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT