mac80211: minstrel_ht: improve ampdu length estimation

If the driver does not report A-MPDU length, estimate it based on the rate.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-6-nbd@nbd.name


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