ieee802154: ca8210: fix possible u8 overflow in ca8210_rx_done

gcc warning this:

drivers/net/ieee802154/ca8210.c:730:10: warning:
 comparison is always false due to limited range of data type [-Wtype-limits]

'len' is u8 type, we get it from buf[1] adding 2, which can overflow.
This patch change the type of 'len' to unsigned int to avoid this,also fix
the gcc warning.

Fixes: ded845a ("ieee802154: Add CA8210 IEEE 802.15.4 device driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>

drivers/net/ieee802154/ca8210.c

@@ -721,7 +721,7 @@ static void ca8210_mlme_reset_worker(struct work_struct *work)
 static void ca8210_rx_done(struct cas_control *cas_ctl)
 {
 	u8 *buf;
-	u8 len;
+	unsigned int len;
 	struct work_priv_container *mlme_reset_wpc;
 	struct ca8210_priv *priv = cas_ctl->priv;
 
@@ -730,7 +730,7 @@ static void ca8210_rx_done(struct cas_control *cas_ctl)
 	if (len > CA8210_SPI_BUF_SIZE) {
 		dev_crit(
 			&priv->spi->dev,
-			"Received packet len (%d) erroneously long\n",
+			"Received packet len (%u) erroneously long\n",
 			len
 		);
 		goto finish;