sam0/uart: remove useless read-modify-write op
sam0/uart: remove useless read-modify-write op

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