speexenc: fix tautology
speexenc: fix tautology

Checking for upper or lower case RIFF fourcc makes sense, checking
twice for the same one does not.

felipebetancur/speex,mwgoldsmith/speex,mwgoldsmith/speex,mwgoldsmith/speex,felipebetancur/speex,felipebetancur/speex,mwgoldsmith/speex,felipebetancur/speex