wil6210: fix wrong index in wil_vring_free

When destroying Rx vring, branch for Rx used wrong Tx descriptor:
while SW context was taken for "head", HW descriptor was, by mistake,
taken from "tail"
Signed-off-by: NVladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

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

@@ -133,7 +133,7 @@ static void wil_vring_free(struct wil6210_priv *wil, struct vring *vring,
 		} else { /* rx */
 			struct vring_rx_desc dd, *d = ⅆ
 			volatile struct vring_rx_desc *_d =
-					&vring->va[vring->swtail].rx;
+					&vring->va[vring->swhead].rx;
 
 			*d = *_d;
 			pa = wil_desc_addr(&d->dma.addr);