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>