leds: lm3642: Fix up world writable sysfs files

We don't need these sysfs files to be world writable or group writable.
These files are write-only, change them 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>