it87: No sysfs files for disabled fans

Only create the fan attributes for enabled fan tachometers. Some
motherboards have a nice BIOS which only enables the fan inputs which
are wired to a fan header on the board. This makes the configuration
easier for the user.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>