mac80211: consider QoS Null frames for STA_NULLFUNC_ACKED

Commit 7b6ddeaf ("mac80211: use QoS NDP for AP probing")
let STAs send QoS Null frames as PS triggers if the AP was
a QoS STA.  However, the mac80211 PS stack relies on an
interface flag IEEE80211_STA_NULLFUNC_ACKED for
determining trigger frame ACK, which was not being set for
acked non-QoS Null frames. The effect is an inability to
trigger hardware sleep via IEEE80211_CONF_PS since the QoS
Null frame was seemingly never acked.

This bug only applies to drivers which set both
IEEE80211_HW_REPORTS_TX_ACK_STATUS and
IEEE80211_HW_PS_NULLFUNC_STACK.

Detect the acked QoS Null frame to restore STA power save.

Fixes: 7b6ddeaf

 ("mac80211: use QoS NDP for AP probing")
Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20191119053538.25979-4-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>