sh_eth: Implement multicast statistic based on the RFS8 status bit

At least on the R8A7790, RFS8 reflects the RINT8 (multicast) MAC
status flag.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>