cpu/sam0_common: uart: fix baud rate calculation
cpu/sam0_common: uart: fix baud rate calculation

In fractional mode, 3 bits are used to store the fractional part.
Therefore we must multiply / divide by 8 instead of 10 in order to
get the correct values.

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