ASoC: tegra20-ac97: add missing clk_disable_unprepare() on error path

Add the missing clk_disable_unprepare() before return from
tegra20_ac97_platform_probe() in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>