drivers/mtd_mapper: fix read_page and write_page backend
drivers/mtd_mapper: fix read_page and write_page backend

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