cryptodev: fix doxygen comment of bulk alloc

cryptodev: fix doxygen comment of bulk alloc

When calling rte_crypto_op_bulk_alloc, the function may
return either a 0, if not enough objects are available
in the mempool or the number of operations requested,
it there are enough available. However, the Doxygen comments
were not matching these two cases.

Fixes: c0f87eb5252b ("cryptodev: change burst API to be crypto op oriented")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Fiona Trahe <fiona.trahe@intel.com>