Add comment to explain iridium exception in the mavlink module

src/modules/mavlink/mavlink_main.cpp

@@ -609,6 +609,10 @@ Mavlink::mavlink_open_uart(const int baud, const char *uart_name, const bool for
 		}
 	}
 
+	/*
+	 * Return here in the iridium mode since the iridium driver does not
+	 * support the subsequent function calls.
+	*/
 	if (_uart_fd < 0 || _mode == MAVLINK_MODE_IRIDIUM) {
 		return _uart_fd;
 	}