cpu/sam0_common: DAC: wait for DAC to be ready
cpu/sam0_common: DAC: wait for DAC to be ready

The DAC can have some start-up delay.
If we try to write to it before it's ready, it will get stuck.

This happens now that `tests/driver_dac_dds` immediately sets a DAC
value after init.

The samd2x class of MCUs doesn't have this bit, but a quick test on
samd10 shows that it might not be nececary there - the DAC does not
get stuck when writing to it immediately after init.

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