ASoC: codecs: Replace instances of rtd->codec with dai->codec

With CODEC to CODEC links rtd->codec does not necessarily point to the driver's
CODEC. CODEC drivers should always use dai->codec and never even look at the PCM
runtime.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>