rc/dsm: decode improvements

- always check system field for validity
 - reject any data outside of "servo position" valid range from Spektrum specification
 - properly support XPlus channels (12+)
 - debug message if channel count changes

src/modules/rc_update/rc_update.cpp

@@ -320,6 +320,13 @@ RCUpdate::Run()
 
 	if (_input_rc_sub.copy(&rc_input)) {
 
+		// warn if the channel count is changing (possibly indication of error)
+		if (!rc_input.rc_lost && (_channel_count_previous != rc_input.channel_count) && (_channel_count_previous > 0)) {
+			PX4_DEBUG("RC channel count changed %d -> %d", _channel_count_previous, rc_input.channel_count);
+		}
+
+		_channel_count_previous = rc_input.channel_count;
+
 		/* detect RC signal loss */
 		bool signal_lost = true;
 

src/modules/rc_update/rc_update.h

@@ -166,6 +166,8 @@ private:
 
 	hrt_abstime _last_rc_to_param_map_time = 0;
 
+	uint8_t _channel_count_previous{0};
+
 	perf_counter_t		_loop_perf;			/**< loop performance counter */
 
 	DEFINE_PARAMETERS(