Fix MassStorageKeyboard demo USE_INTERNAL_SERIAL check being performed before the required library headers were included, causing a compilation error.

Added notes to the class driver functions indicating which functions require what Device/Host state machine states to function.

LUFA/Drivers/USB/Class/Host/HID.c

@@ -220,6 +220,9 @@ uint8_t HID_Host_SendReportByID(USB_ClassInfo_HID_Host_t* const HIDInterfaceInfo
                                 void* Buffer, const uint16_t ReportSize)
 {
 #if !defined(HID_HOST_BOOT_PROTOCOL_ONLY)
+	if ((USB_HostState != HOST_STATE_Configured) || !(HIDInterfaceInfo->State.IsActive))
+	  return false;
+
 	if (HIDInterfaceInfo->State.DeviceUsesOUTPipe)
 	{
 		uint8_t ErrorCode;