drivers/periph_cpuid: Minor cleanup
drivers/periph_cpuid: Minor cleanup

Avoid use of `memcpy` altogether to work around false positive of
`-Warray-bounds` in newer GCC versions.

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