p2p: fix value of DiscSubprotocolError

We had the wrong value (12) since forever.

p2p/peer_error.go

@@ -66,7 +66,7 @@ const (
 	DiscUnexpectedIdentity
 	DiscSelf
 	DiscReadTimeout
-	DiscSubprotocolError
+	DiscSubprotocolError = 0x10
 )
 
 var discReasonToString = [...]string{