ASoC: Make return type of dpcm_state_string() const char *

dpcm_state_string() returns a pointer to a string literal. Modifying a
string literal causes undefined behaviour. So make the return type of the
function const char * to make it explicit that the returned value should
not be modified.

This patch is purely cosmetic, none of the users of dpcm_state_string()
attempt to modify the returned content.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-pcm.c

@@ -2882,7 +2882,7 @@ int snd_soc_platform_trigger(struct snd_pcm_substream *substream,
 EXPORT_SYMBOL_GPL(snd_soc_platform_trigger);
 
 #ifdef CONFIG_DEBUG_FS
-static char *dpcm_state_string(enum snd_soc_dpcm_state state)
+static const char *dpcm_state_string(enum snd_soc_dpcm_state state)
 {
 	switch (state) {
 	case SND_SOC_DPCM_STATE_NEW: