sh_eth: WARN on access to a register not implemented in a particular chip

Currently we may silently read/write a register at offset 0.  Change
this to WARN and then ignore the write or read-back all-ones.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>