ASoC: wm8994: Fix class W controls

Commit 6e0650 (ASoC: wm8994: Use SOC_SINGLE_EXT() instead of open-coding
it) went too far and converted a DAPM control to use SOC_SINGLE_EXT()
which crashes.  Revert that portion of the patch.

Signed-off-by: Mark Brown <broonie@linaro.org>