hwmon/abituguru: Fix unchecked return status

Fix an unused return value warning for the abituguru driver.
Also make sure the sysfs files are created before we register with
the hwmon class, and delete the sysfs files on driver removal.

Signed-off-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>