ASoC: core: Remove duplicated rtd->codec initialization

rtd->codec is already initialized in soc_bind_dai_link(), so there is no need to
do it again in soc_dai_link_init(). Removing the rtd->codec initialization from
soc_dai_link_init() also removes the need for soc_dai_link_init() to know about
the CODEC at all.

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