hwmon: (nct6775) Only display fan speed tolerance conditionally

A fan speed tolerance only makes sense if a fan target speed has been
configured in the first place. Otherwise we get odd output such as

fan1_target:0
fan1_tolerance:337500

Only display values other than 0 if a fan target speed has been configured.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>