eventdev: fix build

eventdev: fix build

build error:
.../lib/librte_eventdev/rte_event_eth_tx_adapter.c:
  In function ‘txa_service_queue_del’:
.../lib/librte_eventdev/rte_event_eth_tx_adapter.c:800:7:
  error: ‘ret’ may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
compilation terminated due to -Wfatal-errors.

https://mails.dpdk.org/archives/test-report/2018-October/065919.html

'ret' may be used uninitialized when 'dev->data->nb_tx_queues' is 0,
although this is not a practical value, initialize 'ret' to cover this
case.

Fixes: a3bbf2e09756 ("eventdev: add eth Tx adapter implementation")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>