rc80211_minstrel: fix contention window calculation

The contention window is supposed to be a power of two minus one, i.e.
15, 31, 63, 127...  minstrel_rate_init() forgets to subtract 1, so the
sequence becomes 15, 32, 66, 134...

Bug reported by Dan Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel.c

@@ -418,7 +418,7 @@ minstrel_rate_init(void *priv, struct ieee80211_supported_band *sband,
 
 			/* contention window */
 			tx_time_single += t_slot + min(cw, mp->cw_max);
-			cw = (cw + 1) << 1;
+			cw = (cw << 1) | 1;
 
 			tx_time += tx_time_single;
 			tx_time_cts += tx_time_single + mi->sp_ack_dur;