Fixes lost RC on px4fmu-v4

A recent change in in the fmu stops the instance when a
   xxxx_reset command is used issued if the fmu was not
   already running.

   That change left publications published and then on the
   next creation created a new publications was created.

   This change calls orb_unadvertise to mark the publication
   as un published so that on the next instantiations of the
   fmu it resumes publishing on the same publication.

src/drivers/px4fmu/fmu.cpp

@@ -1764,6 +1764,11 @@ void PX4FMU::stop()
 	orb_unsubscribe(_armed_sub);
 	orb_unsubscribe(_param_sub);
 
+	orb_unadvertise(_to_input_rc);
+	orb_unadvertise(_outputs_pub);
+	orb_unadvertise(_to_safety);
+	orb_unadvertise(_to_mixer_status);
+
 	/* make sure servos are off */
 	up_pwm_servo_deinit();