drivers/video/backlight/lm3639_bl.c: 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>