cfgfile: fix return value comment

Function rte_cfgfile_load can return NULL value, when something goes wrong.

Signed-off-by: Dmitriy Yakovlev <bombermag@gmail.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>

lib/librte_cfgfile/rte_cfgfile.h

@@ -72,7 +72,7 @@ struct rte_cfgfile_entry {
 * @param flags
 *   Config file flags, Reserved for future use. Must be set to 0.
 * @return
-*   Handle to configuration file
+*   Handle to configuration file on success, NULL otherwise
 */
 struct rte_cfgfile *rte_cfgfile_load(const char *filename, int flags);