mac80211: fix rts threshold check

mac80211: fix rts threshold check

commit a2c40249a36d0b4d76d1caf6bf806e4ae5b06e8a upstream.

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.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>