net: usb: lan78xx: Limit packet length to skb->len

In the Linux kernel, the following vulnerability has been resolved: net: usb: lan78xx: Limit packet length to skb->len Packet length retrieved from descriptor may be larger than the actual socket buffer length. In such case the cloned skb passed up the network stack will leak kernel memory contents. Additionally prevent integer underflow when size is less than ETH_FCS_LEN.