Copter: use rangefinder class's valid_range_count

Also read_rangefinder directly updates rangefinder_alt variable instead of returning distance
No functional change

ArduCopter/ArduCopter.cpp

@@ -614,7 +614,7 @@ void Copter::update_altitude()
     read_barometer();
 
     // read in rangefinder altitude
-    rangefinder_alt = read_rangefinder();
+    read_rangefinder();
 
     // write altitude info to dataflash logs
     if (should_log(MASK_LOG_CTUN)) {

ArduCopter/Copter.h

@@ -946,7 +946,7 @@ private:
     void init_barometer(bool full_calibration);
     void read_barometer(void);
     void init_rangefinder(void);
-    int16_t read_rangefinder(void);
+    void read_rangefinder(void);
     bool rangefinder_alt_ok();
     void init_compass();
     void init_optflow();

ArduCopter/sensors.cpp

@@ -34,7 +34,7 @@ void Copter::init_rangefinder(void)
 }
 
 // return rangefinder altitude in centimeters
-int16_t Copter::read_rangefinder(void)
+void Copter::read_rangefinder(void)
 {
 #if RANGEFINDER_ENABLED == ENABLED
     rangefinder.update();
@@ -42,19 +42,12 @@ int16_t Copter::read_rangefinder(void)
     // exit immediately if rangefinder is disabled
     if (rangefinder.status() != RangeFinder::RangeFinder_Good) {
         rangefinder_alt_health = 0;
-        return 0;
+        return;
     }
 
-    int16_t temp_alt = rangefinder.distance_cm();
+    rangefinder_alt_health = rangefinder.range_valid_count();
 
-    if (temp_alt >= rangefinder.min_distance_cm() &&
-        temp_alt <= rangefinder.max_distance_cm() * RANGEFINDER_RELIABLE_DISTANCE_PCT) {
-        if (rangefinder_alt_health < RANGEFINDER_HEALTH_MAX) {
-            rangefinder_alt_health++;
-        }
-    }else{
-        rangefinder_alt_health = 0;
-    }
+    int16_t temp_alt = rangefinder.distance_cm();
 
  #if RANGEFINDER_TILT_CORRECTION == 1
     // correct alt for angle of the rangefinder
@@ -63,9 +56,10 @@ int16_t Copter::read_rangefinder(void)
     temp_alt = (float)temp_alt * temp;
  #endif
 
-    return temp_alt;
+    rangefinder_alt = temp_alt;
 #else
-    return 0;
+    rangefinder_alt_health = 0;
+    rangefinder_alt = 0;
 #endif
 }