hwmon: (it87) Use single group and is_visible for miscellaneous attributes

Use is_visible to determine if attributes should be generated or not.
This simplifies the code and reduces object size by about 120 bytes
on x86_64.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>