DVDPlayerAudio: Present correct channel count

Fixes: first part of http://forum.xbmc.org/showthread.php?tid=204518
I looked into DVDAudioCodec and it is some inconsistent.
virtual int GetEncodedChannels() { return 0; }
virtual int GetEncodedSampleRate() { return GetSampleRate(); }

If it returned GetChannels for GetEncodedChannels we could skip the check here.