mac80211: add ieee80211_hw flag for QoS NDP support

Commit 7b6ddeaf ("mac80211: use QoS NDP for AP probing") added an
argument qos_ok to ieee80211_nullfunc_get to support QoS NDP. Despite
the claim in the commit log "Change all the drivers to *not* allow
QoS NDP for now, even though it looks like most of them should be OK
with that", this commit enables QoS NDP in response to beacons (see
change to mlme.c:ieee80211_send_nullfunc), causing ath9k_htc to lose
IP connectivity. See:
https://patchwork.kernel.org/patch/10241109/
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=891060



Introduce a hardware flag to allow such buggy drivers to override the
correct default behaviour of mac80211 of sending QoS NDP packets.

Signed-off-by: Ben Caradoc-Davies <ben@transient.nz>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>