PixHawk Pro: Update mag IDs to have external higher priority

src/drivers/lis3mdl/lis3mdl.cpp

@@ -941,6 +941,11 @@ LIS3MDL::collect()
 	new_report.temperature = report.t;
 	new_report.temperature = 25 + (report.t / (16 * 8.0f));
 
+	// XXX revisit for SPI part, might require a bus type IOCTL
+
+	unsigned dummy;
+	sensor_is_onboard = !_interface->ioctl(MAGIOCGEXTERNAL, dummy);
+
 	/*
 	 * RAW outputs
 	 *