mempool: fix get function documentation

[ upstream commit 9b3233ab09ea331dcf5fb38d215d91818bef3b47 ]

static function `rte_mempool_do_generic_get()` returns zero on success,
not >=0 as its function comment documents.

Since this function called by public API, the comment causes confusion
on the public API return value.

Fixing the internal function documentation for return value.

Fixes: af75078 ("first public release")

Reported-by: Mahesh Adulla <mahesh.adulla@amd.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@amd.com>
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Huisong Li <lihuisong@huawei.com>

lib/librte_mempool/rte_mempool.h

@@ -1402,7 +1402,7 @@ rte_mempool_put(struct rte_mempool *mp, void *obj)
  * @param cache
  *   A pointer to a mempool cache structure. May be NULL if not needed.
  * @return
- *   - >=0: Success; number of objects supplied.
+ *   - 0: Success.
  *   - <0: Error; code of driver dequeue function.
  */
 static __rte_always_inline int