cpu/sam0_common: SPI: cleanup
cpu/sam0_common: SPI: cleanup

Insert whitespace and factor out functions to make it easier to
add functionality.

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