prism54: prism54_get_encode() test below 0 on unsigned index

previously in this function:

u32 index = (dwrq->flags & IW_ENCODE_INDEX) - 1;

index is unsigned, so if -1, the original test (below) didn't work.
Signed-off-by: NRoel Kluin <12o3l@tiscali.nl>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/prism54/isl_ioctl.c

@@ -1186,7 +1186,7 @@ prism54_get_encode(struct net_device *ndev, struct iw_request_info *info,
 	rvalue |= mgt_get_request(priv, DOT11_OID_DEFKEYID, 0, NULL, &r);
 	devindex = r.u;
 	/* Now get the key, return it */
-	if ((index < 0) || (index > 3))
+	if (index == -1 || index > 3)
 		/* no index provided, use the current one */
 		index = devindex;
 	rvalue |= mgt_get_request(priv, DOT11_OID_DEFKEYX, index, NULL, &r);