mac80211: improve AQL aggregation estimation for low data rates

Links with low data rates use much smaller aggregates and are much more
sensitive to latency added by bufferbloat.
Tune the assumed aggregation length based on the tx rate duration.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20200821163045.62140-3-nbd@nbd.name


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