staging: greybus: audio: constify snd_soc_dai_ops structures

snd_soc_dai_ops are not supposed to change at runtime. All functions
working with snd_soc_dai_ops provided by <sound/soc-dai.h> work with
const snd_soc_dai_ops. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>