ASoC: wm5100/wm8903/wm8996: Replace open-coded snd_soc_dapm_to_codec()

We now have a generic helper function to cast from a DAPM context to a CODEC.
Make use of it in the places which previously open-coded it.

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