AP: Use QoS nullfunc for connection poll

AP: Use QoS nullfunc for connection poll

When polling a station that has been inactive for a while, hostapd currently
always uses a null data frame. This is a bit strange with uAPSD clients
(though it seems to mostly work) since the EOSP bit can never be set in a
non-QoS frame. Make hostapd use QoS null data frames for probing when the
station is a QoS STA.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>