idpf: use desc_ring when checking completion queue DMA allocation

idpf_compl_queue uses a union for comp, comp_4b, and desc_ring. The
release path should check complq->desc_ring to determine whether the DMA
descriptor ring is allocated. The current check against comp works but is
leftover from a previous commit and is misleading in this context.

Switching the check to desc_ring improves readability and more directly
reflects the intended meaning, since desc_ring is the field representing
the allocated DMA-backed descriptor ring.

No functional change.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://patch.msgid.link/20251125223632.1857532-9-anthony.l.nguyen@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: aloktiwa

drivers/net/ethernet/intel/idpf/idpf_txrx.c

@@ -134,7 +134,7 @@ static void idpf_compl_desc_rel(struct idpf_compl_queue *complq)
 {
 	idpf_xsk_clear_queue(complq, VIRTCHNL2_QUEUE_TYPE_TX_COMPLETION);
 
-	if (!complq->comp)
+	if (!complq->desc_ring)
 		return;
 
 	dma_free_coherent(complq->netdev->dev.parent, complq->size,