ASoC: wm8{350,753,971}: Use snd_soc_dapm_to_codec() instead of dapm->codec

The CODEC struct in the snd_soc_dapm_context struct is deprecated and
scheduled for removal. Use the snd_soc_dapm_to_codec() function instead.

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