ASoC: ab8500-codec: Move codec ops on a separate structure

Define ab8500 codec operations structure on its own rather than inline
with snd_soc_dai_drivers to clean up the code and make the style
coherent with other codec drivers.

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>