fix: remove duplicate pid_ref from DMMotorInstance

DMMotorInstance had its own pid_ref field while motor_controller already
has pid_ref. Use motor_controller.pid_ref uniformly.

modules/motor/DMmotor/dmmotor.c

@@ -188,7 +188,7 @@ DMMotorInstance *DMMotorInit(DM_Motor_Init_Config_s *config)
 
 void DMMotorSetRef(DMMotorInstance *motor, float ref)
 {
-    motor->pid_ref = ref;
+    motor->motor_controller.pid_ref = ref;
 }
 
 void DMMotorSetCtrlMode(DMMotorInstance *motor, DM_Motor_Mode_e mode)
@@ -218,7 +218,7 @@ void DMMotorControl(void)
         /* angle PID: IMU feedback → velocity output */
         float pid_ref = PIDCalculate(&motor->motor_controller.angle_PID,
                                      *motor->motor_controller.other_angle_feedback_ptr,
-                                     motor->pid_ref);
+                                     motor->motor_controller.pid_ref);
 
         if (motor->output_reverse == OUTPUT_DIRECTION_REVERSE)
             pid_ref *= -1;

modules/motor/DMmotor/dmmotor.h

@@ -57,7 +57,6 @@ typedef struct
     float mit_kp;                     // MIT mode KP (motor internal position gain, 0~500)
     float mit_kd;                     // MIT mode KD (motor internal velocity gain, 0~5)
 
-    float pid_ref;                    // reference input from application
 } DMMotorInstance;
 
 /* DM motor init config */