M4A tag for the track number is not parsed correctly

When copying the track number tag from the .m4a music file to the newly created .ogg file, the tag "trkn" is not parsed correctly.
faad returns a tuple for 'tracknumber' as (track_number,disc_number)

I include a quick patch to solve this problem, but there should be a better way to fix this.
I'm also not sure if this is the correct way to send a patch (first time sending one, yey!)