rndis_wlan: fix checking for default value

Thresholds uses -1 to indicate that default value should be used.
Since thresholds are unsigned sign checking makes no sense.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wireless/rndis_wlan.c

@@ -1236,7 +1236,7 @@ static int set_rts_threshold(struct usbnet *usbdev, u32 rts_threshold)
 
 	netdev_dbg(usbdev->net, "%s(): %i\n", __func__, rts_threshold);
 
-	if (rts_threshold < 0 || rts_threshold > 2347)
+	if (rts_threshold == -1 || rts_threshold > 2347)
 		rts_threshold = 2347;
 
 	tmp = cpu_to_le32(rts_threshold);