rtl818x: Prevent using not initialized queues

Using not existing queues can panic the kernel with rtl8180/rtl8185 cards.
Ignore the skb priority for those cards, they only have one tx queue. Pierre
Asselin (pa@panix.com) reported the kernel crash in the Gentoo forum:

https://forums.gentoo.org/viewtopic-t-1147832-postdays-0-postorder-asc-start-25.html

He also confirmed that this patch fixes the issue. In summary this happened:

After updating wpa_supplicant from 2.9 to 2.10 the kernel crashed with a
"divide error: 0000" when connecting to an AP. Control port tx now tries to
use IEEE80211_AC_VO for the priority, which wpa_supplicants starts to use in
2.10.

Since only the rtl8187se part of the driver supports QoS, the priority
of the skb is set to IEEE80211_AC_BE (2) by mac80211 for rtl8180/rtl8185
cards.

rtl8180 is then unconditionally reading out the priority and finally crashes on
drivers/net/wireless/realtek/rtl818x/rtl8180/dev.c line 544 without this
patch:
	idx = (ring->idx + skb_queue_len(&ring->queue)) % ring->entries

"ring->entries" is zero for rtl8180/rtl8185 cards, tx_ring[2] never got
initialized.

Cc: stable@vger.kernel.org
Reported-by: pa@panix.com
Tested-by: pa@panix.com
Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220422145228.7567-1-alexander@wetzel-home.de

drivers/net/wireless/realtek/rtl818x/rtl8180/dev.c

@@ -460,8 +460,10 @@ static void rtl8180_tx(struct ieee80211_hw *dev,
 	struct rtl8180_priv *priv = dev->priv;
 	struct rtl8180_tx_ring *ring;
 	struct rtl8180_tx_desc *entry;
+	unsigned int prio = 0;
 	unsigned long flags;
-	unsigned int idx, prio, hw_prio;
+	unsigned int idx, hw_prio;
+
 	dma_addr_t mapping;
 	u32 tx_flags;
 	u8 rc_flags;
@@ -470,6 +472,8 @@ static void rtl8180_tx(struct ieee80211_hw *dev,
 	/* do arithmetic and then convert to le16 */
 	u16 frame_duration = 0;
 
+	/* rtl8180/rtl8185 only has one useable tx queue */
+	if (dev->queues > IEEE80211_AC_BK)
 		prio = skb_get_queue_mapping(skb);
 	ring = &priv->tx_ring[prio];