hwmon: (stts751) Fix buffer size passed to snprintf

Function snprintf already cares for the terminating NUL at the end of
the string, the caller doesn't need to do it.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Andrea Merello <andrea.merello@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>