net: fec: Enable imx6 enet checksum acceleration.

Enables hardware generation of IP header and
protocol specific checksums for transmitted
packets.

Enabled hardware discarding of received packets with
invalid IP header or protocol specific checksums.

The feature is enabled by default but can be
enabled/disabled by ethtool.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>