ASoC: Add helper function to cast component back to CODEC

Add a helper function to cast back from a component struct to the CODEC struct
it is embedded in. This is useful in situations where we know that a certain
component is a CODEC and want to get access to some CODEC specific properties.

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