HID: logitech: Prevent possibility of infinite loop when using /sys interface

If the device data is not accessible for some reason, returning 0 will cause the call to be
continuously called again as none of the string has been 'consumed'.

Signed-off-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>