Motors: Use _motor_to_channel_map in tricopter consistently.

In AP_MotorsTri.cpp the AP_MOTORS_MOT_1, _2 and _4 constants are
always mapped to actual output channels through _motor_to_channel_map
while the _CH_TRI_YAW is not, but there were a few inconsistencies
in this that could lead to things like PWM min and max values being
set on wrong channels.

It looks like all in all _motor_to_channel_map being in PROGMEM
probably doesn't help save memory and I'm not sure how useful it is
in the first place but regardless the usage should be consistent.

libraries/AP_Motors/AP_MotorsHexa.h

@@ -19,7 +19,7 @@ public:
     AP_MotorsHexa(RC_Channel& rc_roll, RC_Channel& rc_pitch, RC_Channel& rc_throttle, RC_Channel& rc_yaw, uint16_t loop_rate, uint16_t speed_hz = AP_MOTORS_SPEED_DEFAULT) : AP_MotorsMatrix(rc_roll, rc_pitch, rc_throttle, rc_yaw, loop_rate, speed_hz) {
     };
 
-    // setup_motors - configures the motors for a quad
+    // setup_motors - configures the motors for a hexa
     virtual void        setup_motors();
 
 protected:

libraries/AP_Motors/AP_MotorsOcta.h

@@ -19,7 +19,7 @@ public:
     AP_MotorsOcta(RC_Channel& rc_roll, RC_Channel& rc_pitch, RC_Channel& rc_throttle, RC_Channel& rc_yaw, uint16_t loop_rate, uint16_t speed_hz = AP_MOTORS_SPEED_DEFAULT) : AP_MotorsMatrix(rc_roll, rc_pitch, rc_throttle, rc_yaw, loop_rate, speed_hz) {
     };
 
-    // setup_motors - configures the motors for a quad
+    // setup_motors - configures the motors for an octa
     virtual void        setup_motors();
 
 protected:

libraries/AP_Motors/AP_MotorsTri.cpp

@@ -77,7 +77,7 @@ void AP_MotorsTri::output_min()
     hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_1]), _rc_throttle.radio_min);
     hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_2]), _rc_throttle.radio_min);
     hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_4]), _rc_throttle.radio_min);
-    hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_CH_TRI_YAW]), _rc_yaw.radio_trim);
+    hal.rcout->write(AP_MOTORS_CH_TRI_YAW, _rc_yaw.radio_trim);
 }
 
 // get_motor_mask - returns a bitmask of which outputs are being used for motors or servos (1 means being used)
@@ -85,7 +85,10 @@ void AP_MotorsTri::output_min()
 uint16_t AP_MotorsTri::get_motor_mask()
 {
     // tri copter uses channels 1,2,4 and 7
-    return (1U << 0 | 1U << 1 | 1U << 3 | 1U << AP_MOTORS_CH_TRI_YAW);
+    return (1U << pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_1])) |
+        (1U << pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_2])) |
+        (1U << pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_4])) |
+        (1U << AP_MOTORS_CH_TRI_YAW);
 }
 
 // output_armed - sends commands to the motors
@@ -226,7 +229,7 @@ void AP_MotorsTri::output_test(uint8_t motor_seq, int16_t pwm)
             break;
         case 3:
             // back servo
-            hal.rcout->write(pgm_read_byte(&_motor_to_channel_map[AP_MOTORS_MOT_7]), pwm);
+            hal.rcout->write(AP_MOTORS_CH_TRI_YAW, pwm);
             break;
         case 4:
             // front left motor

libraries/AP_Motors/AP_MotorsY6.h

@@ -21,7 +21,7 @@ public:
     AP_MotorsY6(RC_Channel& rc_roll, RC_Channel& rc_pitch, RC_Channel& rc_throttle, RC_Channel& rc_yaw, uint16_t loop_rate, uint16_t speed_hz = AP_MOTORS_SPEED_DEFAULT) : AP_MotorsMatrix(rc_roll, rc_pitch, rc_throttle, rc_yaw, loop_rate, speed_hz) {
     };
 
-    // setup_motors - configures the motors for a quad
+    // setup_motors - configures the motors for a Y6
     virtual void        setup_motors();
 
 protected: