Extend SERIAL_CHAR to take multiple arguments

Marlin/src/gcode/config/M221.cpp

@@ -39,8 +39,7 @@ void GcodeSuite::M221() {
   }
   else {
     SERIAL_ECHO_START();
-    SERIAL_CHAR('E');
-    SERIAL_CHAR('0' + target_extruder);
+    SERIAL_CHAR('E', '0' + target_extruder);
     SERIAL_ECHOPAIR(" Flow: ", planner.flow_percentage[target_extruder]);
     SERIAL_CHAR('%');
     SERIAL_EOL();