hwmon: (tmp421) Convert to use devm_hwmon_device_register_with_groups

Use devm_hwmon_device_register_with_groups API to attach attributes
to hwmon device, simplify code, and reduce code size.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>