leds: triggers: send uevent when changing triggers

Some triggers create sysfs files when they are enabled.  Send a uevent
"change" notification whenever the trigger is changed to allow userspace
processes such as udev to modify permissions on the new files.

Change-Id: I31410cbc0f5c146f31c61326f8006e98b3b7201e
Signed-off-by: Colin Cross <ccross@android.com>