AP_ADC: fixed APM1 HIL_SENSORS Ch6() return

libraries/AP_ADC/AP_ADC_HIL.cpp

@@ -60,7 +60,10 @@ uint32_t AP_ADC_HIL::Ch6(const uint8_t *channel_numbers, float *result)
     for (uint8_t i=0; i<6; i++) {
         result[i] = Ch(channel_numbers[i]);
     }
-    return ((hal.scheduler->millis() - last_hil_time)*2)/5;
+    uint32_t now = hal.scheduler->micros();
+    uint32_t ret = now - _last_ch6_time;
+    _last_ch6_time = now;
+    return ret;
 }
 
 // Set one channel value

libraries/AP_ADC/AP_ADC_HIL.h

@@ -56,8 +56,8 @@ private:
     // The raw adc array
     uint16_t        adcValue[8];
 
-    // the time in milliseconds when we last got a HIL update
-    uint32_t        last_hil_time;
+    // the time in microseconds when Ch6 was last requested
+    uint32_t        _last_ch6_time;
 
     ///
     // sensor constants