mac80211: calculate beacon loss time accurately

Instead of using a fixed 2 second timeout, calculate beacon loss interval
from the advertised beacon interval and a frame count.  With this beacon
loss happens after N (default 7) consecutive frames are missed which
for a typical setup (100TU beacon interval) is ~700ms (or ~1/3 previous).

Signed-off-by: Sam Leffler <sleffler@chromium.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc net/mac80211/ieee80211_i.h
Simple merge

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