hwmon: (w83793) Saving negative errors in unsigned

"ret" is used to store the return value for watchdog_trigger() and it
should be signed for the error handling to work.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>