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>
diff --git a/sound/soc/codecs/wm8994.c b/sound/soc/codecs/wm8994.c
index 24131a7..c99b6da 100644
--- a/sound/soc/codecs/wm8994.c
+++ b/sound/soc/codecs/wm8994.c
@@ -1433,7 +1433,7 @@
 
 #define WM8994_CLASS_W_SWITCH(xname, reg, shift, max, invert) \
 	SOC_SINGLE_EXT(xname, reg, shift, max, invert, \
-		snd_soc_get_volsw, wm8994_put_class_w)
+		snd_soc_dapm_get_volsw, wm8994_put_class_w)
 
 static int wm8994_put_class_w(struct snd_kcontrol *kcontrol,
 			      struct snd_ctl_elem_value *ucontrol)