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 --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
index 3598abf..ff7bc30 100644
--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -349,6 +349,7 @@
 	struct work_struct chswitch_work;
 	struct work_struct beacon_connection_loss_work;
 
+	unsigned long beacon_timeout;
 	unsigned long probe_timeout;
 	int probe_send_count;