eventdev: fix eth Tx adapter queue count checks

rte_event_eth_tx_adapter_queue_add() - add a check
that returns an error if the ethdev has zero Tx queues
configured.

rte_event_eth_tx_adapter_queue_del() - remove the
checks for ethdev queue count, instead check for
queues added to the adapter which maybe different
from the current ethdev queue count.

Fixes: a3bbf2e09756 ("eventdev: add eth Tx adapter implementation")
Cc: stable@dpdk.org
Signed-off-by: Nikhil Rao <nikhil.rao@intel.com>