mac80211: optimize station connection monitor

Calling mod_timer for every rx/tx packet can be quite expensive.
Instead of constantly updating the timer, we can simply let it run out
and check the timestamp of the last ACK or rx packet to re-arm it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20200908123702.88454-9-nbd@nbd.name


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