mac80211: add standard deviation to Minstrel stats

This patch adds the statistical descriptor "standard deviation"
to better describe the current properties of Minstrel and
Minstrel-HTs success probability distribution. The standard
deviation (SD) is calculated as exponential weighted moving
standard deviation (EWMSD) and its current value is added as
new column in all rc_stats (in debugfs).

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc net/mac80211/rc80211_minstrel.c
Simple merge

diff --cc net/mac80211/rc80211_minstrel.h
Simple merge

diff --cc net/mac80211/rc80211_minstrel_debugfs.c
Simple merge

diff --cc net/mac80211/rc80211_minstrel_ht_debugfs.c
Simple merge