ASoC: tegra: Remove redundant initialisation of compat_filter_fn

Setting a field in a static struct to NULL has no effect so don't bother
(and don't generate false positives for grep).

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Stephen Warren <swarren@nvidia.com>