mac80211: minstrel_ht: rename prob_ewma to prob_avg, use it for the new average

Reduces per-rate data structure size

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20191008171139.96476-3-nbd@nbd.name


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