drivers/video/backlight/lm3630a_bl.c: fix signedness bug in lm3630a_chip_init()

"rval" needs to be signed for the error handling to work.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>