sh_eth: offload RX checksum on R8A7740

The R-Mobile A1 (R8A7740) 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>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>