hwmon: (gpio-fan) Use is_visible to determine if attributes should be created

Simplify code and reduce object size by more than 300 bytes (x86_64).

Cc: Jamie Lentin <jm@lentin.co.uk>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>