fix logger: sscanf was used wrong for custom topics file

using scanf with %s reads until the first whitespace, which included the
comma (as per C standard and tested on linux). Behavior on NuttX differs.

This makes it work on both Linux and Nuttx.

src/modules/logger/logger.h

@@ -99,7 +99,7 @@ public:
 	 * (because it does not write an ADD_LOGGED_MSG message).
 	 * @param name topic name
 	 * @param interval limit rate if >0, otherwise log as fast as the topic is updated.
-	 * @return 0 on success
+	 * @return -1 on error, file descriptor otherwise
 	 */
 	int add_topic(const char *name, unsigned interval);