ASoC: tegra: add function to set ac97 rate

AC97 uses a fixed rate, unrelated to the sample rate. Add a function to
make the setup more trivial.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>