ASoC: Drop const from struct snd_soc_dai_link *of_node members

Dropping the const qualifiers prevents "passing argument 1 of ‘of_node_put’
discards ‘const’ qualifier from pointer target type" type warnings when
compiling the code dropping reference to cpu_of_node, codec_of_node or
platform_of_node with with an of_node_put() function call.

This lets us to avoid casting to struct device_node * or caching variables
internally in drivers just to be able to properly drop a reference to the
OF node on clean up paths.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>