backlight: ld9040: replace EFAULT with EINVAL

Replace EFAULT with EINVAL, because EFAULT tends to be for the invalid
memory addresses.

Signed-off-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>