Fix SDL audio playback with surround sound

If user has surround sound enabled, ioq3 would not play any sound.
Fix painting sound buffer for 4/5.1 audio channels. Extra channels
currently play no audio.

code/client/snd_dma.c

@@ -1339,7 +1339,7 @@ void S_Update_(void) {
 		& ~(dma.submission_chunk-1);
 
 	// never mix more than the complete buffer
-	samps = dma.samples >> (dma.channels-1);
+	samps = dma.samples / dma.channels;
 	if (endtime - s_soundtime > samps)
 		endtime = s_soundtime + samps;