ASoC: dapm: Fix build warning

commit c6615082

 ("ASoC: dapm: add code to configure dai link
parameters") introduced the following build warning:

sound/soc/soc-dapm.c: In function 'snd_soc_dapm_new_pcm':
sound/soc/soc-dapm.c:3389:4: warning: passing argument 1 of 'snprintf'
discards 'const' qualifier from pointer target type
snprintf(w_param_text[count], len,

This patch fixes this by switching to using devm_kasprintf. This also
saves a couple of lines of code.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>