mac80211: factor out code to look up the average packet length duration for a rate

This will be used to enhance AQL estimated aggregation length

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20200821163045.62140-2-nbd@nbd.name


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