wspr: use *vdl.Value instead of interface{} for decoding.

Change-Id: I1208eaa890b2190e7f96e2424b15ca2b25dec007
diff --git a/services/wsprd/app/app.go b/services/wsprd/app/app.go
index d03cb7c..4fc2d15 100644
--- a/services/wsprd/app/app.go
+++ b/services/wsprd/app/app.go
@@ -489,10 +489,12 @@
 
 	inArgs := make([]interface{}, msg.NumInArgs)
 	for i := range inArgs {
-		if err := decoder.Decode(&inArgs[i]); err != nil {
+		var v *vdl.Value
+		if err := decoder.Decode(&v); err != nil {
 			w.Error(err)
 			return
 		}
+		inArgs[i] = v
 	}
 
 	request := &outstandingRequest{