conf: pcm dmix - add CHANNELS argument

It seems that some audio devices do use only mono audio for some
applications (RPi).

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

src/conf/pcm/dmix.conf

@@ -3,7 +3,7 @@
 #
 
 pcm.!dmix {
-	@args [ CARD DEV SUBDEV FORMAT RATE ]
+	@args [ CARD DEV SUBDEV FORMAT RATE CHANNELS ]
 	@args.CARD {
 		type string
 		default {
@@ -36,6 +36,13 @@ pcm.!dmix {
 			name defaults.pcm.dmix.rate
 		}
 	}
+	@args.CHANNELS {
+		type integer
+		default {
+			@func refer
+			name defaults.pcm.dmix.channels
+		}
+	}
 	type dmix
 	ipc_key {
 		@func refer
@@ -58,6 +65,7 @@ pcm.!dmix {
 		}
 		format $FORMAT
 		rate $RATE
+		channels $CHANNELS
 		period_size {
 			@func refer
 			name {