base-files: disable LEDs if default state is undefined

base-files: disable LEDs if default state is undefined

Set the default state for LEDs to off. When a trigger is set, the
trigger will turn the LED automatically on.

Currently LEDs might stay on, e.g. when the LED trigger is set to a
netdev trigger and the interface is never activated or the 'none'
trigger is selected without setting the 'default' option to 0 and it's
set for the LED indicating the system running state.

Using off as a default value is also consistent with the documentation
in the OpenWrt wiki.

Signed-off-by: David Bauer <mail@david-bauer.net>