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>