timer: clarify error if subsystem already initialized

rte_timer_subsystem_init() may return -EALREADY if it has been already
initialized. Therefore put explicitly into doxygen that this is not a
failure for the application.

Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Reviewed-by: Michal Krawczyk <mk@semihalf.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>

lib/librte_timer/rte_timer.h

@@ -171,6 +171,7 @@ int rte_timer_data_dealloc(uint32_t id);
  *   - 0: Success
  *   - -ENOMEM: Unable to allocate memory needed to initialize timer
  *      subsystem
+ *   - -EALREADY: timer subsystem was already initialized. Not an error.
  */
 int rte_timer_subsystem_init(void);