ASoC: zx-i2s: constify snd_soc_dai_ops structure

This structure is only stored in the ops field of a snd_soc_dai_driver
structure. That field is declared const, so snd_soc_dai_ops structures
that have this property can be declared as const also.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>