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/iwlegacy/common.c b/drivers/net/wireless/iwlegacy/common.c
index b42052b..e5ac047 100644
--- a/drivers/net/wireless/iwlegacy/common.c
+++ b/drivers/net/wireless/iwlegacy/common.c
@@ -5355,7 +5355,7 @@
 	if (changes & BSS_CHANGED_ASSOC) {
 		D_MAC80211("ASSOC %d\n", bss_conf->assoc);
 		if (bss_conf->assoc) {
-			il->timestamp = bss_conf->timestamp;
+			il->timestamp = bss_conf->last_tsf;
 
 			if (!il_is_rfkill(il))
 				il->ops->post_associate(il);