tracing/ftrace: adapt mmiotrace to the new type of print_line, fix

Correct the value's type of trace_empty function
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -1750,7 +1750,7 @@ static int trace_empty(struct trace_iterator *iter)
 		}
 	}
 
-	return TRACE_TYPE_HANDLED;
+	return 1;
 }
 
 static enum print_line_t print_trace_line(struct trace_iterator *iter)