drivers/leds/leds-asic3: make LEDS_ASIC3 depend on LEDS_CLASS

We call led_classdev_unregister/led_classdev_register in
asic3_led_remove/asic3_led_probe, thus make LEDS_ASIC3 depend on
LEDS_CLASS.

This patch fixes below build error if LEDS_CLASS is not configured.

    LD      .tmp_vmlinux1
  drivers/built-in.o: In function `asic3_led_remove':
  clkdev.c:(.devexit.text+0x1860): undefined reference to `led_classdev_unregister'
  drivers/built-in.o: In function `asic3_led_probe':
  clkdev.c:(.devinit.text+0xcee8): undefined reference to `led_classdev_register'
  make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Paul Parsons <lost.distance@yahoo.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>