ASoC: tegra: Use NULL instead of 0 for pointers

Fixes the following sparse warnings:
sound/soc/tegra/tegra30_ahub.c:583:16: warning:
Using plain integer as NULL pointer
sound/soc/tegra/tegra30_ahub.c:600:16: warning:
Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>