sh_eth: offload RX checksum on R8A77980

The R-Car V3H (R8A77980) SoC manual describes the Ether MAC's RX checksum
offload the same way as it's implemented in the EtherAVB MAC...

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