ASoC: fsl_sai: 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 of the rates array from u32 to
unsigned int. While both resolve to the same type, u32 is usually used to
empathize that the value is for a 32-bit hardware register or similar which
makes it slightly confusing in this context.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>