ks8695net: Disable non-working ethtool operations

Some ethtool operations can only be implemented for the WAN port, and
not all such operations are allowed to return an error code such as
-EOPNOTSUPP.  Therefore, define two separate ethtool_ops structures
for WAN and non-WAN ports; simplify and rename the WAN-only functions.

This is completely untested as I don't have an ARM build environment.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>