drivers/video/backlight/adp5520_bl.c: fix compiler warning in adp5520_show()

While compiling with make W=1 (gcc gcc (GCC) 4.7.2 20121109 (Red Hat
4.7.2-8)) the following warning happens:

  drivers/video/backlight/adp5520_bl.c: In function `adp5520_show':
  drivers/video/backlight/adp5520_bl.c:146:6: warning: variable `error' set but not used [-Wunused-but-set-variable]

fixed by checking the return value of the variable

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Acked-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>