sh_eth: offload RX checksum on R7S72100

The RZ/A1H (R7S721000) 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>