Function udev_device_get_sysattr_value 

pub fn udev_device_get_sysattr_value(
    dev: &mut UdevDevice,
    sysattr: &str,
) -> Option<String>

Gets the sys attribute file value.

From the libudev documentation:

The retrieved value is cached in the device. Repeated calls will return the same
value and not open the attribute again.

Returns: the content of a sys attribute file, or None if there is no sys attribute value.