mac80211: Use suitable semantics for beacon availability indication

Currently beacon availability upon association is marked by have_beacon
flag of assoc_data structure that becomes unavailable when association
completes. However beacon availability indication is required also after
association to inform a driver. Currently dtim_period parameter is used
for this purpose. Move have_beacon flag to another structure, persistant
throughout a interface's life cycle. Use suitable sematics for beacon
availability indication.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
[fix another instance of BSS_CHANGED_DTIM_PERIOD in docs]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 89a8377..5a6c135 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -1584,8 +1584,9 @@
 				   BSS_CHANGED_ARP_FILTER |
 				   BSS_CHANGED_PS;
 
-			if (sdata->u.mgd.dtim_period)
-				changed |= BSS_CHANGED_DTIM_PERIOD;
+			/* Re-send beacon info report to the driver */
+			if (sdata->u.mgd.have_beacon)
+				changed |= BSS_CHANGED_BEACON_INFO;
 
 			sdata_lock(sdata);
 			ieee80211_bss_info_change_notify(sdata, changed);