sfc: Convert to use hwmon_device_register_with_groups

Simplify the code. Avoid race conditions caused by attributes
being created after hwmon device registration. Implicitly
(through hwmon API) add mandatory 'name' sysfs attribute.

Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>