[rbp] Fix for audiotoggledigital on Pi.

Pi only has two audio output modes so the three way toggle gets stuck. This fix corresponds to Pi specific settings code:
https://github.com/xbmc/xbmc/blob/master/xbmc/settings/GUISettings.cpp#L458
Merge pull request #2252 from popcornmix/toggle_digital_analogue

[rbp] Fix for audiotoggledigital on Pi