ASoC: Fix snd_soc_kcontrol_platform() return type

This should obviously be snd_soc_platform * and not snd_soc_codec *

Fixes: f6272ff8 ("ASoC: Add snd_soc_kcontrol_platform() helper function")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>