leds: use default-on trigger for Cobalt Qube

This patch changes the default trigger from "ide-disk"
to "default-on". Users updating from kernels not having this
LED driver will prefer having the same LED behavior as they
used to.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>