drivers/video/backlight/lm3639_bl.c: return proper error in lm3639_bled_mode_store() error paths

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>