ath5k: beacon interval is in TU

the beacon interval is passed by mac80211 in TU already, so we can directly use
it without conversion. also update the comments about TU (1 TU is defined by
802.11 as 1024usec).

drivers/net/wireless/ath5k/ath5k.h:     Changes-licensed-under: ISC
drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD
drivers/net/wireless/ath5k/base.h:      Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath5k/ath5k.h b/drivers/net/wireless/ath5k/ath5k.h
index 878609f..c79066b 100644
--- a/drivers/net/wireless/ath5k/ath5k.h
+++ b/drivers/net/wireless/ath5k/ath5k.h
@@ -486,8 +486,8 @@
  * TSF to TU conversion:
  *
  * TSF is a 64bit value in usec (microseconds).
- * TU is a 32bit value in roughly msec (milliseconds): usec / 1024
- * (1000ms equals 976 TU)
+ * TU is a 32bit value and defined by IEEE802.11 (page 6) as "A measurement of
+ * time equal to 1024 usec", so it's roughly milliseconds (usec / 1024).
  */
 #define TSF_TO_TU(_tsf) (u32)((_tsf) >> 10)