ASoC: tegra: fix compile warning in AC'97 driver

This fixes the following by deleting dead code:

sound/soc/tegra/tegra20_ac97.c: In function ‘tegra20_ac97_platform_probe’:
sound/soc/tegra/tegra20_ac97.c:435:1: warning: label ‘err_unregister_pcm’ defined but not used [-Wunused-label]

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>