mac80211: fix WMM AC default for non-QoS data frames

The WMM AC selection added to the monitor mode selection function
accidentally assigns non-QoS data frames to the same AC as mgmt frames
(VO). This is not serious, but should be fixed anyway. This patch
assigns them to the BE AC instead.

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