eventdev: fix possible uninitialized variable

When compiling with -O1, this error can appear:
	lib/librte_eventdev/rte_event_eth_tx_adapter.c:705:6: error:
	‘ret’ may be used uninitialized in this function

If tx_queue_id is -1 and nb_queues is 0, then ret is returned
without being initialized.
It is fixed by setting 0 as initial value.

Fixes: a3bbf2e097 ("eventdev: add eth Tx adapter implementation")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>

lib/librte_eventdev/rte_event_eth_tx_adapter.c

@@ -702,7 +702,7 @@ txa_service_queue_add(uint8_t id,
 	struct txa_service_queue_info *tqi;
 	struct rte_eth_dev_tx_buffer *tb;
 	struct txa_retry *txa_retry;
-	int ret;
+	int ret = 0;
 
 	txa = txa_service_id_to_data(id);