Minor changes to tx_callback

Finalize the error procedure that was accidentally deleted.

host/hackrf-tools/src/hackrf_transfer.c

@@ -416,7 +416,8 @@ int tx_callback(hackrf_transfer* transfer) {
 		} else {
 			return 0;
 		}
-
+	} else {
+        return -1;
     }
 }