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/drivers/net/wireless/rt2x00/rt2x00config.c b/drivers/net/wireless/rt2x00/rt2x00config.c
index d7c0f86..293676b 100644
--- a/drivers/net/wireless/rt2x00/rt2x00config.c
+++ b/drivers/net/wireless/rt2x00/rt2x00config.c
@@ -102,7 +102,7 @@
 
 	/* Update the AID, this is needed for dynamic PS support */
 	rt2x00dev->aid = bss_conf->assoc ? bss_conf->aid : 0;
-	rt2x00dev->last_beacon = bss_conf->timestamp;
+	rt2x00dev->last_beacon = bss_conf->last_tsf;
 
 	/* Update global beacon interval time, this is needed for PS support */
 	rt2x00dev->beacon_int = bss_conf->beacon_int;