nl80211: fix code calculating average signal and rate

Using average of previous average and the next value is highly
imprecise. E.g. for values 20, 20, 20, 180 it would result in /average/
of 100 (instead of 60). Fix it by storing & using an info of how many
samples were used for the previous calculation.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>