mac80211: minstrel_ht: fix rounding issue in MCS duration calculation

On very high MCS bitrates, the calculated duration of rates that are
next to each other can be very imprecise, due to the small packet size
used as reference (1200 bytes).
This is most visible in VHT80 nss=2 MCS8/9, for which minstrel shows the
same throughput when the probability is also the same. This leads to a
bad rate selection for such rates.

Fix this issue by introducing an average A-MPDU size factor into the
calculation.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>