ath10k: fix error return code

Return a negative error code on failure.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
identifier ret; expression e1,e2;
@@
(
if (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath10k/htt_tx.c

@@ -475,8 +475,10 @@ int ath10k_htt_tx(struct ath10k_htt *htt, struct sk_buff *msdu)
 
 	skb_cb->htt.txbuf = dma_pool_alloc(htt->tx_pool, GFP_ATOMIC,
 					   &paddr);
-	if (!skb_cb->htt.txbuf)
+	if (!skb_cb->htt.txbuf) {
+		res = -ENOMEM;
 		goto err_free_msdu_id;
+	}
 	skb_cb->htt.txbuf_paddr = paddr;
 
 	skb_cb->paddr = dma_map_single(dev, msdu->data, msdu->len,