ASoC: tegra: Staticize some functions in tegra30_i2s.c

'tegra30_i2s_startup' and 'tegra30_i2s_shutdown' are used only in this file and
hence made static. Fixes the following sparse warnings:
sound/soc/tegra/tegra30_i2s.c:74:5: warning:
symbol 'tegra30_i2s_startup' was not declared. Should it be static?
sound/soc/tegra/tegra30_i2s.c:101:6: warning:
symbol 'tegra30_i2s_shutdown' was not declared. Should it be static?

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>