ethtool: ethtool_get_drvinfo: populate drvinfo fields even if callback exits

ethtool: ethtool_get_drvinfo: populate drvinfo fields even if callback exits

If ethtool_ops::get_drvinfo() callback isn't set,
ethtool_get_drvinfo() will fill the ethtool_drvinfo::name and
ethtool_drvinfo::bus_info fields.

However, if the driver provides the callback function, those two
fields are not touched. This means that the driver has to fill these
itself.

Allow the driver to leave those two fields empty and populate them in
such case. This way, the driver can rely on the default values for the
name and the bus_info. If the driver provides values, do nothing.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/r/20221108035754.2143-1-mailhol.vincent@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>