ASoC: wm8994: Use SOC_ENUM_SINGLE_VIRT_DECL()

For the upcoming consolidation for MUXs and virtual MUXs we need to mark virtual
enums as such.
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>

sound/soc/codecs/wm8994.c

@@ -1344,8 +1344,7 @@ static const char *adc_mux_text[] = {
 	"DMIC",
 };
 
-static SOC_ENUM_SINGLE_DECL(adc_enum,
-			    0, 0, adc_mux_text);
+static SOC_ENUM_SINGLE_VIRT_DECL(adc_enum, adc_mux_text);
 
 static const struct snd_kcontrol_new adcl_mux =
 	SOC_DAPM_ENUM_VIRT("ADCL Mux", adc_enum);