sh_eth: R8A7740 supports packet shecksumming

The R8A7740 GEther controller supports the packet checksum offloading
but the 'hw_crc' (bad name, I'll fix it) flag isn't set in the R8A7740
data,  thus CSMR isn't cleared...

Fixes: 73a0d907 ("net: sh_eth: add support R8A7740")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>