ASoC: soc.h: makes snd_soc_of_parse_audio_prefix() inline

commit 3b710356 ("ASoC: soc-core: add
snd_soc_of_parse_node_prefix()") maked snd_soc_of_parse_audio_prefix()
as #define. But it'd be better to make this a static inline rather than
a #define. It helps with error messages and type safety.
This patch makes it inline.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>