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 ([email protected]) 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: [email protected]
Reported-by: [email protected]
Tested-by: [email protected]
Signed-off-by: Alexander Wetzel <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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