conf: Allow 2.1 surround to use different number of channels

This way, cards that support LFE on four channels (e g laptop with
internal subwoofer) can do that, and other cards on a six channel setup
can use that as well.

Well, note that there is still a reference to "pcm.surround51" left here.
In practice, for HDA Intel sound cards this does not matter as both
surround51 and surround40 reference the same definition.
(And that's the only card I currently know of that actually does
surround2.1 over four channels.)

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

src/conf/pcm/surround21.conf

@@ -51,10 +51,9 @@ pcm.!surround21 {
 			]
 		}
 	}
-	slave.channels 6
-	ttable.0.0 1
-	ttable.1.1 1
-	ttable.2.5 1
+	ttable.0.FL 1
+	ttable.1.FR 1
+	ttable.2.LFE 1
 	hint {
 		description "2.1 Surround output to Front and Subwoofer speakers"
 		device $DEV