While an empty led_sysfs definition works as intended (no LED state is
touched), an error message is generated, since the appropriate sysfs
interface does not exist:

          Sat Feb 27 16:42:55 2016 user.err wifitoggle: led:  not found

This patch makes sure that no such error message will be generated.

Signed-off-by: Karol Babioch <karol@babioch.de>