backlight: fix ot200_bl build

Fix build error when GPIO_CS5535 is not enabled:

  drivers/built-in.o: In function `ot200_backlight_probe':
  ot200_bl.c:(.text+0x205bf): undefined reference to `cs5535_gpio_set'
  ot200_bl.c:(.text+0x205d1): undefined reference to `cs5535_gpio_set'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Christian Gmeiner <christian.gmeiner@gmail.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>