cpu/sam0_common: fix potential undefined result with sercom_id
cpu/sam0_common: fix potential undefined result with sercom_id

Scan-build detected that sercom_id could return -1 and the value of this function is affected to uint8_t variables. Since these variables are used for shitfing bit in registers, this could lead to undefined behavior

aeneby/RIOT,mtausig/RIOT,josephnoir/RIOT,smlng/RIOT,ant9000/RIOT,toonst/RIOT,mtausig/RIOT,x3ro/RIOT,jasonatran/RIOT,aeneby/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,smlng/RIOT,yogo1212/RIOT,yogo1212/RIOT,authmillenon/RIOT,josephnoir/RIOT,jasonatran/RIOT,toonst/RIOT,kYc0o/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,basilfx/RIOT,RIOT-OS/RIOT,mtausig/RIOT,toonst/RIOT,mtausig/RIOT,miri64/RIOT,josephnoir/RIOT,OlegHahm/RIOT,x3ro/RIOT,ant9000/RIOT,kaspar030/RIOT,smlng/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,aeneby/RIOT,kaspar030/RIOT,OTAkeys/RIOT,yogo1212/RIOT,basilfx/RIOT,jasonatran/RIOT,basilfx/RIOT,authmillenon/RIOT,basilfx/RIOT,toonst/RIOT,yogo1212/RIOT,x3ro/RIOT,yogo1212/RIOT,ant9000/RIOT,x3ro/RIOT,kYc0o/RIOT,toonst/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,josephnoir/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,x3ro/RIOT,basilfx/RIOT,smlng/RIOT,ant9000/RIOT,miri64/RIOT,smlng/RIOT,yogo1212/RIOT,OlegHahm/RIOT,mtausig/RIOT,kaspar030/RIOT,aeneby/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,josephnoir/RIOT,ant9000/RIOT,aeneby/RIOT,OlegHahm/RIOT