ASoC: doc: Fix undefined SND_SOC_DAPM_NOPM argument

The virtual widget example makes use of an undefined SND_SOC_DAPM_NOPM
argument passed to SND_SOC_DAPM_MIXER().  Replace with the correct
SND_SOC_NOPM definition.
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20231121120751.77355-1-cristian.ciocaltea@collabora.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/sound/soc/dapm.rst

@@ -234,7 +234,7 @@ corresponding soft power control. In this case it is necessary to create
 a virtual widget - a widget with no control bits e.g.
 ::
 
-  SND_SOC_DAPM_MIXER("AC97 Mixer", SND_SOC_DAPM_NOPM, 0, 0, NULL, 0),
+  SND_SOC_DAPM_MIXER("AC97 Mixer", SND_SOC_NOPM, 0, 0, NULL, 0),
 
 This can be used to merge to signal paths together in software.