When ieee80211_monitor_select_queue encounters data frames, it selects
the WMM AC based on skb->priority and assumes that skb->priority
contains a valid 802.1d tag. However this assumption is incorrect, since
ieee80211_select_queue has not been called at this point.
If skb->priority > 7, an array overrun occurs, which could lead to
invalid values, resulting in crashes in the tx path.
Fix this by setting skb->priority based on the 802.11 header for QoS
frames and using the default AC for all non-QoS frames.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>