ASoC: rx1950_uda1380: Constify rate constraints

The rate constraints in this board driver are not modified at runtime, so
make them const.

While we are at it also remove the unnecessary 0 initializer for the mask
field.

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