HAL_SITL: take account of available bytes in receive_time_constraint_us

libraries/AP_HAL_SITL/UARTDriver.cpp

@@ -518,12 +518,12 @@ void UARTDriver::_timer_tick(void)
   
   A return value of zero means the HAL does not support this API
 */
-uint64_t UARTDriver::receive_time_constraint_us(uint16_t nbytes) const
+uint64_t UARTDriver::receive_time_constraint_us(uint16_t nbytes)
 {
     uint64_t last_receive_us = _receive_timestamp;
     if (_uart_baudrate > 0) {
         // assume 10 bits per byte. 
-        uint32_t transport_time_us = (1000000UL * 10UL / _uart_baudrate) * nbytes;
+        uint32_t transport_time_us = (1000000UL * 10UL / _uart_baudrate) * (nbytes+available());
         last_receive_us -= transport_time_us;
     }
     return last_receive_us;

libraries/AP_HAL_SITL/UARTDriver.h

@@ -80,7 +80,7 @@ public:
 
       A return value of zero means the HAL does not support this API
      */
-    uint64_t receive_time_constraint_us(uint16_t nbytes) const override;
+    uint64_t receive_time_constraint_us(uint16_t nbytes) override;
     
 private:
     uint8_t _portNumber;