AP_MotorsHeli: add range checking to ext_gyro_gain

libraries/AP_Motors/AP_MotorsHeli_Single.h

@@ -79,8 +79,8 @@ public:
     //  this can be used to ensure other pwm outputs (i.e. for servos) do not conflict
     uint16_t get_motor_mask() override;
 
-    // ext_gyro_gain - set external gyro gain in range 0 ~ 1
-    void ext_gyro_gain(float gain)  override { _ext_gyro_gain_std = gain * 1000.0f; }
+    // ext_gyro_gain - set external gyro gain in range 0 ~ 1000
+    void ext_gyro_gain(float gain)  override { if (gain >= 0 && gain <= 1000) { _ext_gyro_gain_std = gain; }}
 
     // has_flybar - returns true if we have a mechical flybar
     bool has_flybar() const  override { return _flybar_mode; }