ASoC: cs42l73: Constify rate constraints

The rate constraints in this driver are shared between all device instances. It
should not be (and is not) modified at runtime, so make them const.

While we are at it also change the type for the rates array from u32 to unsigned
int.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>