Add motor off PWM value

src/drivers/drv_pwm_output.h

@@ -78,6 +78,11 @@ __BEGIN_DECLS
  */
 #define PWM_LOWEST_MIN 90
 
+/**
+ * Default value for a shutdown motor
+ */
+#define PWM_MOTOR_OFF	800
+
 /**
  * Default minimum PWM in us
  */