cpu/stm32/periph_eth: improve debugging output
cpu/stm32/periph_eth: improve debugging output

Add ENABLE_DEBUG_VERBOSE flag, so that the noise during debugging can be
reduced. This is super helpful when testing under load, as otherwise there is
just too much noise in the output.

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