ignore MDL-ERROR-IND from LAPD layer

The MDL-ERROR is already logged in lapd_core.  Let's avoid printing
"Unhandled prim" log lines when receiving it in osmo-v5

src/v5x_protocol.c

@@ -428,6 +428,10 @@ int v5x_dl_rcv(struct osmo_dlsap_prim *odp, uint16_t dladdr, void *rx_cbdata)
 			break;
 		}
 		goto out;
+	case PRIM_MDL_ERROR:
+		/* already generates a log message in lapd_core.c, suppress the below log line */
+		rc = 0;
+		goto out;
 	default:
 		LOGV5IF(v5if, DV5, LOGL_NOTICE, "Unhandled prim=%d (ldaddr=%d)\n", odp->oph.primitive, dladdr);
 		rc = -EINVAL;