ASoC: sound/ad73311: add missing __devexit marker

This fixes the following warning:

sound/soc/codecs/ad73311.c:50:12: warning: 'ad73311_remove' defined but not used

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>