Sanity check SERIAL_PORT

Addressing #7400

Marlin/src/inc/SanityCheck.h

   #error "SERIAL_XON_XOFF and SERIAL_STATS_* features not supported on USB-native AVR devices."
 #endif
 
+#if SERIAL_PORT > 7
+  #error "Set SERIAL_PORT to the port on your board. Usually this is 0."
+#endif
+
 /**
  * Dual Stepper Drivers
  */