ath9k_htc: use non-QoS NDP for AP probing

When switching mac80211 to use QoS NDP, it turned out that
ath9k_htc is somehow broken by this, e.g. see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=891060

.

Fix this by using the new mac80211 flag to go back to the
old, incorrect, behaviour for this driver.

Fixes: 7b6ddeaf ("mac80211: use QoS NDP for AP probing")
Reported-by: Ben Caradoc-Davies <ben@transient.nz>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>