leds: lm355x: Fix up world writable sysfs file

We don't need the sysfs file to be world writable or group writable.
This file is write-only, change it to S_IWUSR (0200).

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: G.Shark Jeong <gshark.jeong@gmail.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>