hwmon: (i5500_temp) Don't bind to disabled sensors

On many motherboards, for an unknown reason, the thermal sensor seems
to be disabled and will return a constant temperature value of 36.5
degrees Celsius. Don't bind to the device in that case, so that we
don't report this bogus value to userspace.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Romain Dolbeau <romain@dolbeau.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>