sh_eth: offload RX checksum on SH7763

The SH7763 SoC manual describes the Ether MAC's RX checksum offload
the same way as it's implemented in the EtherAVB MACs...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>