netdev: bfin_mac: deduce Ethernet FCS from hardware IP payload checksum

IP checksum is based on 16-bit one's complement algorithm, so to deduce a
value from checksum is equal to add its complement.

Unfortunately, the Blackfin on-chip MAC checksum logic only works when the
IP packet has a header length of 20 bytes.  This is true for most IPv4
packets, but not for IPv6 packets or IPv4 packets which use header options.
So only use the hardware checksum when appropriate.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jon Kowal <jon.kowal@dspecialists.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>