ASoC: wm97xx: fix compilation corner case

When the old AC97 is not used, CONFIG_SND_SOC_AC97_BUS is not
defined. As a consequence, in the error path, snd_soc_free_ac97_codec()
is not defined and triggers a compilation error.

Fix it for wm9705 and wm9712, as wm9713 is correctly written.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>