drivers/pwm_out: set default failsafe values

src/drivers/drv_pwm_output.h

@@ -86,6 +86,11 @@ struct pwm_output_values {
  */
 #define PWM_MOTOR_OFF	900
 
+/**
+ * Default value for a servo stop
+ */
+#define PWM_SERVO_STOP	1500
+
 /**
  * Default minimum PWM in us
  */

src/drivers/pwm_out/PWMOut.cpp

@@ -613,6 +613,14 @@ void PWMOut::update_params()
 						int32_t pwm_fail_new = _mixing_output.failsafeValue(i);
 						param_set(param_find(str), &pwm_fail_new);
 					}
+
+				} else {
+					if (pwm_default_channel_mask & 1 << i) {
+						_mixing_output.failsafeValue(i) = PWM_MOTOR_OFF;
+
+					} else {
+						_mixing_output.failsafeValue(i) = PWM_SERVO_STOP;
+					}
 				}
 
 			} else {