leds: triggers: return error if invalid trigger name is provided via sysfs

If an invalid trigger name is provided via sysfs currently no error
is returned. Therefore it's not possible to determine whether the
trigger was set successfully.
Fix this by returning -EINVAL if no trigger is matched.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>