drivers/video/backlight/adp8860_bl.c: remove a redundant assignment for max_brightness

We have set props.max_brightness before registering backlight device.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>