wil6210: fix dma mapping error cleanup in __wil_tx_vring_tso

In case we fail to map one of the TSO SKB fragments, we need to
clear all the mapped descriptors, from swhead to swhead+descs_used-1.

Change the desc index calculation to
i = (swhead + descs_used - 1) % vring->size;
to prevent unmpping of (swhead + descs_used) descriptor that wasn't
mapped.
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/wil6210/txrx.c

@@ -1594,7 +1594,7 @@ static int __wil_tx_vring_tso(struct wil6210_priv *wil, struct vring *vring,
 	while (descs_used > 0) {
 		struct wil_ctx *ctx;
 
-		i = (swhead + descs_used) % vring->size;
+		i = (swhead + descs_used - 1) % vring->size;
 		d = (struct vring_tx_desc *)&vring->va[i].tx;
 		_desc = &vring->va[i].tx;
 		*d = *_desc;