ASoC: Make rtd->codec optional

There are some place in the ASoC core that expect rtd->codec to be non NULL
(mainly CODEC specific sysfs files). With componentization going forward
rtd->codec might be NULL in some cases. This patch prepares the core for this by
not registering CODEC specific sysfs files if rtd->codec is NULL. sysfs file
removal does not need to be conditionalized as it handles the removal of
non-existing files just fine.

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