tg3: Convert to use hwmon_device_register_with_groups

Use new hwmon API to simplify code, provide missing mandatory 'name'
sysfs attribute, and attach hwmon attributes to hwmon device instead
of pci device.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>