ASoC: ac97: fixup multi-platform AC'97 module build failure

commit b047e1cc (ASoC: ac97: Support multi-platform AC'97) introduced
some build failures for modules wanting to access the generic
soc_ac97_ops.  For example:

ERROR: "soc_ac97_ops" [sound/soc/codecs/snd-soc-wm9712.ko] undefined!

To fix, export soc_ac97_ops to modules.

Cc: Mark Brown <broonie@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/soc-core.c

@@ -2080,6 +2080,7 @@ int snd_soc_new_ac97_codec(struct snd_soc_codec *codec,
 EXPORT_SYMBOL_GPL(snd_soc_new_ac97_codec);
 
 struct snd_ac97_bus_ops *soc_ac97_ops;
+EXPORT_SYMBOL_GPL(soc_ac97_ops);
 
 int snd_soc_set_ac97_ops(struct snd_ac97_bus_ops *ops)
 {