ASoC: sound/wm9090: add missing __devexit marker

This fixes the following warning:

sound/soc/codecs/wm9090.c:668:12: warning: 'wm9090_i2c_remove' defined but not used

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