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>