cmocka: volume: Add default case to handle unsupported formats

The switch supporting various sample formats was missing a default handler,
which caused compilation errors when enabling/adding a new format that is
not supported.

Signed-off-by: Adrian Warecki <adrian.warecki@intel.com>

test/cmocka/src/audio/volume/volume_process.c

@@ -278,8 +278,9 @@ static void test_audio_vol(void **state)
 	case SOF_IPC_FRAME_FLOAT:
 		fill_source_s32(vol_state);
 		break;
-	case SOF_IPC_FRAME_S24_3LE:
+
 	/* TODO: add 3LE support */
+	default:
 		break;
 	}