8139cp: fix checksum broken

I am not family with RealTek RTL-8139C+ series 10/100 PCI Ethernet driver.
I try to guess the meaning of RxProtoIP and IPFail.
RxProtoIP stands for received IPv4 packet that upper protocol is not tcp and udp.
!(status & IPFail) is true means that driver correctly to check checksum in IPv4 header.

If these are right, driver will set ip_summed with CHECKSUM_UNNECESSARY for other
upper protocol, e.g. sctp, igmp protocol. This will cause protocol stack ignores
checksum check for packets with invalid checksum.

This patch is only compile-test.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>