sfc/siena: fix null pointer dereference in efx_hard_start_xmit

Like in previous patch for sfc, prevent potential (but unlikely) NULL
pointer dereference.

Fixes: 12804793 ("sfc: decouple TXQ type from label")
Reported-by: Tianhao Zhao <tizhao@redhat.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Link: https://lore.kernel.org/r/20220915141958.16458-1-ihuguet@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/siena/tx.c

@@ -336,7 +336,7 @@ netdev_tx_t efx_siena_hard_start_xmit(struct sk_buff *skb,
 		 * previous packets out.
 		 */
 		if (!netdev_xmit_more())
-			efx_tx_send_pending(tx_queue->channel);
+			efx_tx_send_pending(efx_get_tx_channel(efx, index));
 		return NETDEV_TX_OK;
 	}