When clients are idle for too long, hostapd sends nullfunc frames for
probing. When those are acked by the client, the idle time needs to be
updated.

To make this work (and to avoid unnecessary probing), update sta->last_rx
whenever an ACK was received for a tx packet. Only do this if the flag
IEEE80211_HW_REPORTS_TX_ACK_STATUS is set.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>