ASoC: Constify ops and compr_ops fields of snd_soc_dai_link

The core does not modify these fields, so they can be made const. This allows
drivers to declare their op tables as const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>