drivers/video/backlight/adp8860_bl.c: check strict_strtoul() return value

Handle return value, strict_strtoul is declared with attribute
warn_unused_result.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Acked-by: Richard Purdie <rpurdie@linux.intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>