MCS: use value_string for conversion

Change-Id: I212ebb892ab162821633974d5a6c7e315d308370
diff --git a/src/decoding.cpp b/src/decoding.cpp
index 3410291..1473ff7 100644
--- a/src/decoding.cpp
+++ b/src/decoding.cpp
@@ -370,7 +370,7 @@
 	default:
 		LOGP(DRLCMACDL, LOGL_ERROR,
 			"Decoding of uplink %s data blocks not yet supported.\n",
-			cs.name());
+			mcs_name(cs));
 		return -ENOTSUP;
 	};