net: ena: replace free_tx/rx_ids union with single free_ids field in ena_ring

struct ena_ring holds a union of free_rx_ids and free_tx_ids.
Both of the above fields mean the exact same thing and are used
exactly the same way.
Furthermore, these fields are always used with a prefix of the
type of ring. So for tx it will be tx_ring->free_tx_ids, and for
rx it will be rx_ring->free_rx_ids, which shows how redundant the
"_tx" and "_rx" parts are.
Furthermore still, this may lead to confusing code like where
tx_ring->free_rx_ids which works correctly but looks like a mess.

This commit removes the aforementioned redundancy by replacing the
free_rx/tx_ids union with a single free_ids field.
It also changes a single goto label name from err_free_tx_ids: to
err_tx_free_ids: for consistency with the above new notation.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>