Misc. cleanup, serial strings

Marlin/src/gcode/host/M118.cpp

@@ -57,9 +57,9 @@ void GcodeSuite::M118() {
     if (WITHIN(port, 0, NUM_SERIAL))
       serial_port_index = (
         port == 0 ? SERIAL_BOTH
-        : port == 1 ? SERIAL_PORT
-        #ifdef SERIAL_PORT_2
-          : port == 2 ? SERIAL_PORT_2
+        : port == 1 ? 0
+        #if HAS_MULTI_SERIAL
+          : port == 2 ? 1
         #endif
         : SERIAL_PORT
       );