fmu:Fixes cause of 0 values reported in pwm info

The root cause was the replacment of a local variable num_outputs
   with the class member _num_outputs.
   The effect of a "bad mix" is to return 0 - this clampped the
   _num_outputs to 0.

   Prior to commit  3b3e2b2 px4fmu: "consolidate usage of output mode"
   this would not have  been an issue because the local num_outputs
   was reset every cycle"

  As a secondary issue. We sould call up_pwm_servo_init() to
  establish the PWM channel allocation early. This then allows
  FMU::set_pwm_rate to properly check for improper rate request
  not isolate to one group (timer).

src/drivers/px4fmu/fmu.cpp

@@ -673,6 +673,7 @@ PX4FMU::set_mode(Mode mode)
 		if (old_mask != _pwm_mask) {
 			/* disable servo outputs - no need to set rates */
 			up_pwm_servo_deinit();
+			return OK;
 		}
 
 		break;
@@ -680,8 +681,8 @@ PX4FMU::set_mode(Mode mode)
 	default:
 		return -EINVAL;
 	}
-
 	_mode = mode;
+	up_pwm_servo_init(_pwm_mask);
 	return OK;
 }
 
@@ -1048,7 +1049,6 @@ void
 PX4FMU::update_pwm_out_state(bool on)
 {
 	if (on && !_pwm_initialized && _pwm_mask != 0) {
-		up_pwm_servo_init(_pwm_mask);
 		set_pwm_rate(_pwm_alt_rate_channels, _pwm_default_rate, _pwm_alt_rate);
 		_pwm_initialized = true;
 	}
@@ -1251,7 +1251,7 @@ PX4FMU::cycle()
 
 				/* do mixing */
 				float outputs[_max_actuators];
-				_num_outputs = _mixers->mix(outputs, _num_outputs, NULL);
+				size_t mixed_num_outputs = _mixers->mix(outputs, _num_outputs, NULL);
 
 				/* publish mixer status */
 				multirotor_motor_limits_s multirotor_motor_limits = {};
@@ -1270,7 +1270,7 @@ PX4FMU::cycle()
 
 				/* disable unused ports by setting their output to NaN */
 				for (size_t i = 0; i < sizeof(outputs) / sizeof(outputs[0]); i++) {
-					if (i >= _num_outputs) {
+					if (i >= mixed_num_outputs) {
 						outputs[i] = NAN_VALUE;
 					}
 				}
@@ -1278,26 +1278,26 @@ PX4FMU::cycle()
 				uint16_t pwm_limited[_max_actuators];
 
 				/* the PWM limit call takes care of out of band errors, NaN and constrains */
-				pwm_limit_calc(_throttle_armed, arm_nothrottle(), _num_outputs, _reverse_pwm_mask,
+				pwm_limit_calc(_throttle_armed, arm_nothrottle(), mixed_num_outputs, _reverse_pwm_mask,
 					       _disarmed_pwm, _min_pwm, _max_pwm, outputs, pwm_limited, &_pwm_limit);
 
 
 				/* overwrite outputs in case of force_failsafe with _failsafe_pwm PWM values */
 				if (_armed.force_failsafe) {
-					for (size_t i = 0; i < _num_outputs; i++) {
+					for (size_t i = 0; i < mixed_num_outputs; i++) {
 						pwm_limited[i] = _failsafe_pwm[i];
 					}
 				}
 
 				/* overwrite outputs in case of lockdown with disarmed PWM values */
 				if (_armed.lockdown || _armed.manual_lockdown) {
-					for (size_t i = 0; i < _num_outputs; i++) {
+					for (size_t i = 0; i < mixed_num_outputs; i++) {
 						pwm_limited[i] = _disarmed_pwm[i];
 					}
 				}
 
 				/* output to the servos */
-				for (size_t i = 0; i < _num_outputs; i++) {
+				for (size_t i = 0; i < mixed_num_outputs; i++) {
 					pwm_output_set(i, pwm_limited[i]);
 				}
 
@@ -1309,7 +1309,7 @@ PX4FMU::cycle()
 					up_pwm_update();
 				}
 
-				publish_pwm_outputs(pwm_limited, _num_outputs);
+				publish_pwm_outputs(pwm_limited, mixed_num_outputs);
 				perf_end(_ctl_latency);
 			}
 		}