net: xilinx_emaclite: fix receive buffer overflow

xilinx_emaclite looks at the received data to try to determine the
Ethernet packet length but does not properly clamp it if
proto_type == ETH_P_IP or 1500 < proto_type <= 1518, causing a buffer
overflow and a panic via skb_panic() as the length exceeds the allocated
skb size.

Fix those cases.

Also add an additional unconditional check with WARN_ON() at the end.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Fixes: bb81b2dd ("net: add Xilinx emac lite device driver")
Signed-off-by: David S. Miller <davem@davemloft.net>