ASoC: fix build warning in soc-core.c

Fix kernel-doc build error. A symbol that ends with an underscore
character ('_') has special meaning in reST (reStructuredText), so add
a '*' to prevent this error and to indicate that there are several of
these values to choose from.

../sound/soc/soc-core.c:2799: ERROR: Unknown target name: "snd_soc_daifmt".
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-core.c

@@ -2796,7 +2796,7 @@ EXPORT_SYMBOL_GPL(snd_soc_dai_set_bclk_ratio);
 /**
  * snd_soc_dai_set_fmt - configure DAI hardware audio format.
  * @dai: DAI
- * @fmt: SND_SOC_DAIFMT_ format value.
+ * @fmt: SND_SOC_DAIFMT_* format value.
  *
  * Configures the DAI hardware format and clocking.
  */