[ALSA] ASoC AC97 static GPL symbol fix

This patch fixes a bug whereby the GPL'ed symbol ac97_dai was being
declared as static.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>