drivers/video/backlight/lm3639_bl.c: remove ret = -EIO at error paths of probe

The APIs are returning correctly the err codes, no need to assign -EIO
to the ret again.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Acked-by: Daniel Jeong <daniel.jeong@ti.com>
Cc: G.Shark Jeong <gshark.jeong@gmail.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>