sysfs store functions should return the number of bytes written.
Returning zero results in an endless loop.

Fixes: 08426eda ("hwmon: Add driver for FTS BMC chip "Teutates"")
Signed-off-by: NThilo Cestonaro <thilo.cestonaro@ts.fujitsu.com>
[groeck: Clean up documentation change and description]
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>