mac80211: rename bss_conf timestamp to last_tsf

This value is not really very useful by itself,
yet some drivers (including iwlwifi until I can
figure out what it should do) use it. At least
rename it to "last_tsf" to indicate the meaning
and add a note that it may be really old.

I suspect the value may become useful combined
with the rx_status->mactime, but we don't (yet)
store that value and pass it to the driver.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 44e4dfc..9a012be 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -229,7 +229,8 @@
  *	valid in station mode only while @assoc is true and if also
  *	requested by %IEEE80211_HW_NEED_DTIM_PERIOD (cf. also hw conf
  *	@ps_dtim_period)
- * @timestamp: beacon timestamp
+ * @last_tsf: last beacon's/probe response's TSF timestamp (could be old
+ *	as it may have been received during scanning long ago)
  * @beacon_int: beacon interval
  * @assoc_capability: capabilities taken from assoc resp
  * @basic_rates: bitmap of basic rates, each bit stands for an
@@ -276,7 +277,7 @@
 	u8 dtim_period;
 	u16 beacon_int;
 	u16 assoc_capability;
-	u64 timestamp;
+	u64 last_tsf;
 	u32 basic_rates;
 	int mcast_rate[IEEE80211_NUM_BANDS];
 	u16 ht_operation_mode;