hwmon: (gpio-fan) Change name used in hwmon_device_register_with_groups

Since commit 648cd48c
The hwmon name attributes must not include '-' so the name must be
rename from gpio-fan to gpio_fan

Signed-off-by: Julien D'Ascenzio <jdascenzio@yahoo.fr>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>