wifi: mac80211: cleanup airtime arithmetic with ieee80211_sta_keep_active()

Prefer native jiffies-wide 'unsigned long' for the 'last_active' field of
'struct airtime_info' and introduce 'ieee80211_sta_keep_active()' for airtime
check in 'ieee80211_txq_keep_active()' and 'ieee80211_sta_register_airtime()'.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://msgid.link/20231206060935.612241-1-dmantipov@yandex.ru


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