mac80211: fix rate control initialisation

In commit 2e92e6f2 ("mac80211: use rate
index in TX control") I forgot to initialise a few new variables to -1 which
means that the rate control algorithm is never triggered and 0 is used as
the only rate index, effectively fixing the transmit bitrate at the lowest
supported.

This patch adds the missing initialisation.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Bisected-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -947,6 +947,8 @@ __ieee80211_tx_prepare(struct ieee80211_tx_data *tx,
 	tx->local = local;
 	tx->sdata = IEEE80211_DEV_TO_SUB_IF(dev);
 	tx->channel = local->hw.conf.channel;
+	tx->rate_idx = -1;
+	tx->last_frag_rate_idx = -1;
 	/*
 	 * Set this flag (used below to indicate "automatic fragmentation"),
 	 * it will be cleared/left by radiotap as desired.