drivers/at86rf231: handle channel out of range more conveniently

Changes default behaviour from setting the highest possible channel to returning an error and print a `DEVELHELP` message when applicable. 
Fixes #1670 and change has to be mentioned in the [documentation](https://github.com/RIOT-OS/RIOT/wiki/Board%3A-IoT-LAB_M3#note-on-at86rf231-radio-driver)