ASoC: codecs: Remove rtd->codec usage from CODEC drivers

In order to support CODEC<->CODEC links remove the assumption that there
is only a single CODEC on a DAI link by removing the use of the CODEC
pointer in the rtd from the CODEC drivers. They are already being passed
their DAI whenever they are passed an rtd and can get the CODEC from
there.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>