mac80211: fix rts threshold check

Currently whenever rts thresold is set, every packet will use RTS
protection no matter its size exceeds the threshold or not. This is
due to a bug in the rts threshold check.
	if (len > tx->local->hw.wiphy->rts_threshold) {
		txrc.rts = rts = true;
	}
Basically it is comparing an int (len) and a u32 (rts_threshold),
and the variable len is assigned as:
	len = min_t(int, tx->skb->len + FCS_LEN,
			 tx->local->hw.wiphy->frag_threshold);
However, when frag_threshold is "-1", len is always "-1", which is
0xffffffff therefore rts is always set to true.

CC: stable@kernel.org
Signed-off-by: NShanyu Zhao <shanyu.zhao@intel.com>
Reviewed-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -593,7 +593,8 @@ ieee80211_tx_h_rate_ctrl(struct ieee80211_tx_data *tx)
 	struct ieee80211_hdr *hdr = (void *)tx->skb->data;
 	struct ieee80211_supported_band *sband;
 	struct ieee80211_rate *rate;
-	int i, len;
+	int i;
+	u32 len;
 	bool inval = false, rts = false, short_preamble = false;
 	struct ieee80211_tx_rate_control txrc;
 	u32 sta_flags;
@@ -602,7 +603,7 @@ ieee80211_tx_h_rate_ctrl(struct ieee80211_tx_data *tx)
 
 	sband = tx->local->hw.wiphy->bands[tx->channel->band];
 
-	len = min_t(int, tx->skb->len + FCS_LEN,
+	len = min_t(u32, tx->skb->len + FCS_LEN,
 			 tx->local->hw.wiphy->frag_threshold);
 
 	/* set up the tx rate control struct we give the RC algo */