AP_MotorsMulticopter: throttle_radio_min, max to multicopter class

Moved in from Motors class, only ever used by multicopters
No functional change

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -108,6 +108,8 @@ AP_MotorsMulticopter::AP_MotorsMulticopter(uint16_t loop_rate, uint16_t speed_hz
     _throttle_rpy_mix(AP_MOTORS_THR_LOW_CMP_DEFAULT),
     _min_throttle(AP_MOTORS_DEFAULT_MIN_THROTTLE),
     _hover_out(AP_MOTORS_DEFAULT_MID_THROTTLE),
+    _throttle_radio_min(1100),
+    _throttle_radio_max(1900),
     _batt_voltage_resting(0.0f),
     _batt_current_resting(0.0f),
     _batt_resistance(0.0f),

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -174,6 +174,8 @@ protected:
     float               _throttle_rpy_mix;          // mix between throttle and hover throttle for 0 to 1 and ratio above hover throttle for >1
     int16_t             _min_throttle;              // the minimum throttle to be sent to the motors when they're on (prevents motors stalling while flying)
     int16_t             _hover_out;                 // the estimated hover throttle as pct * 10 (i.e. 0 ~ 1000)
+    int16_t             _throttle_radio_min;        // minimum radio channel pwm
+    int16_t             _throttle_radio_max;        // maximum radio channel pwm
     float               _throttle_thrust_max;       // the maximum allowed throttle thrust 0.0 to 1.0 in the range throttle_min to throttle_max
 
     // spool variables