ethtool: Provide a default implementation of ethtool_ops::get_drvinfo

The driver name and bus address for a net_device can normally be found
through the driver model now.  Instead of requiring drivers to provide
this information redundantly through the ethtool_ops::get_drvinfo
operation, use the driver model to do so if the driver does not define
the operation.  Since ETHTOOL_GDRVINFO no longer requires the driver
to implement any operations, do not require net_device::ethtool_ops to
be set either.

Remove implementations of get_drvinfo and ethtool_ops that provide
only this information.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>