drivers/video/backlight/adp8870_bl.c: fix error return code in adp8870_led_probe()

Fix to return a negative error code from the error handling case instead
of zero, as returned elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
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>