rtlwifi: Add missing DMA buffer unmapping for PCI drivers

In https://bugzilla.kernel.org/show_bug.cgi?id=42976, a system with driver
rtl8192se used as an AP suffers from "Out of SW-IOMMU space" errors. These
are caused by the DMA buffers used for beacons never being unmapped.

This bug was also reported at
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/961618

Reported-and-Tested-by: Da Xue <da@lessconfused.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rtlwifi/pci.c

@@ -912,8 +912,13 @@ static void _rtl_pci_prepare_bcn_tasklet(struct ieee80211_hw *hw)
 	memset(&tcb_desc, 0, sizeof(struct rtl_tcb_desc));
 	ring = &rtlpci->tx_ring[BEACON_QUEUE];
 	pskb = __skb_dequeue(&ring->queue);
-	if (pskb)
+	if (pskb) {
+		struct rtl_tx_desc *entry = &ring->desc[ring->idx];
+		pci_unmap_single(rtlpci->pdev, rtlpriv->cfg->ops->get_desc(
+				 (u8 *) entry, true, HW_DESC_TXBUFF_ADDR),
+				 pskb->len, PCI_DMA_TODEVICE);
 		kfree_skb(pskb);
+	}
 
 	/*NB: the beacon data buffer must be 32-bit aligned. */
 	pskb = ieee80211_beacon_get(hw, mac->vif);