drivers/video/backlight/adp5520_bl.c: use kstrtoul()

The usage of strict_strtoul() is not preferred. Thus, kstrtoul
should be used.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>