Tri: _min_throttle interpreted as 0~1000 range for throttle_lower flag

limit.throttle_lower flag becomes true when the throttle passed into the
motors lib (which is in the 0 ~ 1000 range) is below _min throttle.
This makes the interpretation of the THR_MIN parameter consistent
between the main code (which uses 0 ~ 1000 range) and the motors lib
(which previously used the RC3_MIN ~ RC3_MAX range).
The remaining problem however is that the output of the motors continues
to use THR_MIN as if it were a pwm.  I don't believe this is a dangerous
problem however.

libraries/AP_Motors/AP_MotorsTri.cpp

@@ -99,7 +99,14 @@ void AP_MotorsTri::output_armed()
     limit.throttle_lower = false;
 
     // Throttle is 0 to 1000 only
-    _rc_throttle.servo_out = constrain_int16(_rc_throttle.servo_out, 0, _max_throttle);
+    if (_rc_throttle.servo_out <= 0) {
+        _rc_throttle.servo_out = 0;
+        limit.throttle_lower = true;
+    }
+    if (_rc_throttle.servo_out >= _max_throttle) {
+        _rc_throttle.servo_out = _max_throttle;
+        limit.throttle_upper = true;
+    }
 
     // capture desired roll, pitch, yaw and throttle from receiver
     _rc_roll.calc_pwm();
@@ -120,15 +127,12 @@ void AP_MotorsTri::output_armed()
         motor_out[AP_MOTORS_MOT_2] = _rc_throttle.radio_min + _spin_when_armed_ramped;
         motor_out[AP_MOTORS_MOT_4] = _rc_throttle.radio_min + _spin_when_armed_ramped;
 
-        // Every thing is limited
-        limit.throttle_lower = true;
-
     }else{
         int16_t roll_out            = (float)_rc_roll.pwm_out * 0.866f;
         int16_t pitch_out           = _rc_pitch.pwm_out / 2;
 
         // check if throttle is below limit
-        if (_rc_throttle.radio_out <= out_min) {
+        if (_rc_throttle.servo_out <= _min_throttle) {
             limit.throttle_lower = true;
         }
         //left front