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: ded845a7 ("ieee802154: Add CA8210 IEEE 802.15.4 device driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>